Today, according to Charles Taylor, we find ourselves in a “Secular Age” in which unbelief is a live possibility. Faced with a vocal community of atheists—be they materialists or secular-humanists—the theist is no longer safe to presume that belief in God is a matter of course. As the dominant form of theism in Western civilization, Christianity is the principal target of attacks against belief in God; moreover, the popular objections of atheists tend to conflate theism with Christian fundamentalism. Oftentimes, while a Christian apologist valiantly rebuts atheistic arguments with an array of philosophical proofs, an inconspicuous premise of scientific-rationalism infiltrates the Christian’s first principles, namely, the presupposition that it is irrational to believe in God’s existence if one cannot first impartially prove it with deductive—or at least highly probable inductive—reasoning.
The root of this conviction is as old as Descartes, whose philosophy exclusively employed mathematical, deductive reasoning in order to ensure the certainty of all convictions. Yet, by sacrificing every mode of thought besides mathematical logic to the idol of certainty, Descartes unwittingly subjected a more comprehensive view of human reason to the flames.
To retrieve a truer, more human approach to the question of God’s existence, we must shun the tempting ideal of absolute certainty, which has little to do with lived human experience. In so doing, our conceptions of human knowledge and belief are reoriented, the two being less easy to distinguish in concrete epistemic actions than in philosophical abstraction.
This expanded, more-comprehensive account of human reason allows for the highest form of human knowledge: interpersonal trust. Personal knowledge is precisely what is required in relation to the super-personal (even tri-personal) God; yet it is not enough to speak about God, since the human person is always already addressed by God and must decide how to respond. Theologians rightly speak of the theoretical distinction between the knowledge of God possible to pure, unaided reason (e.g. God is; God is one, etc.) and the knowledge of God that is only possible due to revelation (e.g. God is Trinity; the Word God became man). According to the Christian, however, no human has ever existed who was not addressed by God in the depths of his or her being. Thus, the existential situation of every human being necessitates that human knowledge of God is first and foremost a personal knowledge, which is only secondarily interpreted by philosophical inquiry.
Confronted with the daunting debate over God’s existence, one might seek refuge from doubt in the putative certainty of syllogisms; however, even such deductive logic is in principle susceptible to doubt. Descartes himself wondered, “I sometimes think that others go wrong even when they think they have the most perfect knowledge; so how do I know that I myself don’t go wrong every time I add two and three or count the sides of a square?”
Moreover, logical validity is not a guarantee of truth; premises may ineluctably lead to a conclusion, but a valid syllogism is not true unless all of its premises are true. Because unprovable premises introduce uncertainty, how could one prove the first principles that ground all arguments, such as the principle of noncontradiction? While these remarks in no way disparage the reliability of logical reasoning, they indicate that rationality cannot pull itself up by the bootstraps; rather, a more fundamental assent undergirds all reasoning.
The experience of uncertainty is unavoidable, especially with respect to the fundamental question of God, whose existence transcends our concepts and powers of manipulation. As Ratzinger writes,
In short, there is no escape from the dilemma of being a man. Anyone who makes up his mind to evade the uncertainty of belief will have to experience the uncertainty of unbelief, which can never finally eliminate the possibility that belief may after all be the truth. It is not until belief is rejected that its unrejectability becomes evident.
Likewise, Avery Dulles explains that those who fail to accept this “dilemma of being a man,” wind up “Straining for a kind of evidence that is humanly unobtainable, they fail to see and to respond to the evidences that are at hand. Indubitable certitudes are few and far between.”
Thus, notes Dulles, a natural kind of faith grounds much of what we consider common knowledge:
Good judgment rests upon an incredibly complex process of synthesis and evaluation, of which we can give no full account, but which is not on that score suspect. If implicit reliance on unanalyzed assumptions is to be called “faith,” we shall have to say that practical living is a constant exercise of faith.
Faith, in this sense, is not independent of reason but is rather one aspect of a fuller notion of human reasoning. As Newman clarifies in his Grammar of Assent, the geniuses of history often arrived at their insights through informal inference, which Newman calls the Illative Sense:
The great discoverers of principles . . . have no arguments, no grounds, they see the truth but they do not know how they see it . . . It is the second-rate men who, though most useful in their place, reconcile, finish, and explain.
The “complex process of synthesis and evaluation”—as Dulles puts it—or “informal inference” in Newman’s language, frees reason from its self-imposed shackling to the exhaustively demonstrable; an expansive view of human rationality must attend to the intuitive, imaginative, and unpredictable dynamics of actual human reason.
Paralleling the insights of Newman, Pierre Rousselot identifies operations of reason beyond calculation. In his short yet groundbreaking work, The Eyes of Faith, Rousselot gives an example of two detectives who have before them the same data. Whereas one detective remains dumbfounded, the other sees the data as a clue to a conclusion. Although the insightful detective has no more data before him than his counterpart, he simultaneously sees both the conclusion and the facts qua clues to that conclusion. Rousselot explains, “So the difference between the one who sees and the one who does not does not consist in any difference in the notes of representation, but in the greater or lesser power of their intellectual activity.”
This activity of the mind, he argues, is how we should understand supernatural faith—though this applies also to natural faith—which “accounts for our perceiving the connection, making the synthesis, giving the assent.” In addition to granting the intellect a natural power of synthesis beyond deductive reasoning, Rousselot also argues that the intellect, like the will, is appetitive by nature. Since “intelligence itself is the expression of a natural appetition for the supreme and subsisting Truth” we can no longer pretend that ideal reasoning is impartial and cold; rather, “Love arouses the faculty of knowing, and by the same stroke knowledge justifies that love.” How far we have come from the supposedly impersonal, objective, and unadulterated logic of pure reason! And yet our observations thus far are merely the necessary groundwork, the prolegomena, for discussing belief in God’s existence.
In an informal TV interview, Joseph Ratzinger once said, “I do believe that someone who attempts to prove his way into the faith with pure theory, will not succeed.” Yet how many books by Christian theologians, philosophers, and apologists tout airtight proofs for God’s existence? To be clear, philosophical arguments for God’s existence are extraordinarily valuable and in many ways illustrate theism’s rationality; however, no amount of protest from the Christian can disguise the conception of God antecedent to his arguments.
In other words, an implicit experience of God, and often faith, precedes any philosophical proof for God, since no proofs would be advanced in the first place apart from this prior intuition. An antecedent faith, according to Etienne Gilson, is precisely what has enabled philosophy to discover its limits through history’s prominent theists such as Augustine, Aquinas, Scotus, and countless others.
We do not thereby discredit or undermine the proofs of the Christian philosopher by noting his antecedent conception of God; rather, we are reorienting the approach to these proofs so that the properly secondary role of reflective, philosophical arguments may be evident. As David Walsh argues, philosophical proofs do not prove God’s existence as if from a tabula rasa, a presuppositionless starting-point:
How could we prove a God whom we already know about in advance of our demonstration? The arguments are really ways by which we arrive at that which we know, the God who is called God and who is vastly more than has been glimpsed . . . The question of God comes from God himself. We would be incapable of asking it if we did not already know who it is for whom we search.
Philosophical reflection about God arises out of a prior, fundamental orientation of the human person. Karl Rahner points out that the human is uniquely capable of questioning reality as a whole and his own place within it. This inborn openness to infinity brings the human person before a mystery that can neither be controlled nor totally understood. Rahner writes, “Man is a being oriented toward God. His orientation towards the absolute mystery always continues to be offered to him by this mystery as the ground and content of his being.” We can name the object of this initial experience “God,” but only faith and subsequent reflection can clarify the identity of the One encountered within the human heart and conscience. Yet no amount of reflection or philosophical demonstration can fully account for the prior, implicit knowledge of God: discursive reasoning about God “points to a more original, unthematic and unreflexive knowledge of God.”
St. Thomas was right to assert that all of our knowledge about God is a posteriori, derived from creation as from the effects of a Cause. Thus, Rahner concludes that even initial, unreflective experience and knowledge of God “is mediated by a categorical encounter with concrete reality in our world, both the world of things and the world of persons.” Rahner’s words here align with the common experience of theists, who find indications of God within the natural world rather than in ontological speculations.
Supposing God exists, argues C. Stephen Evans, one would expect that knowledge of him would be accessible to more than those able to string together a syllogism; we should not be surprised if God created us to evolve with an innate tendency to follow “natural signs”—such as the demands of conscience or the contingency of the material world—to the existence of a necessary and good Law-giver and Creator.
Indeed, for most humans this is precisely how God’s existence spontaneously arises. There is nothing unusual or unjustified in concluding that “God exists” after instinctively following signs such as the birth of a child, the love of a spouse, or the majestic view of Mount Rainier. Of course, for many others, no one event can be pointed to nor can their conviction be reduced to a given experience; rather, these individuals have given an assent that is more than the sum of amassed probabilities.
The role of philosophical arguments for them, according to Henri de Lubac, is “simply the elaboration and the rational organization of a permanently subsisting proof, at once simpler and more fundamental, a proof which is natural, spontaneous, and in many cases unformulated but nevertheless inscribed ‘in the deepest recesses of our reasonable nature’.” Philosophical reflection can distinguish, refine, and even confirm, but it cannot negate the perennial human insight into what Rahner calls “the holy mystery.”
To encounter this holy mystery, even if apart from revelation and therefore only vaguely known, is never an experience of something extraneous or irrelevant to ourselves, as would be any fact about the world. The mystery we call “God,” is not a being among others but is the origin and meaning of all else. At all times, God addresses us, calling into question our actions and the direction of our lives. Our experience of God cannot be an impartial observation of a fact or law, no matter how interesting we may regard it. We have no choice but to decide how we respond to God and any response necessarily involves our whole person.
The proper response to God, who touches the deepest regions of our personhood, is one of faith, of belief; but this is where we must be very clear about what we mean. Belief in this context, as Ratzinger explains, “is not at all mere opinion, as we express it in the sentence, ‘I believe the weather will be fine tomorrow.’ It is not doubt; rather it is certainty that God has shown himself and has opened up for us the view of truth itself.” Ratzinger is not contradicting his remark above about the ubiquity of uncertainty, for he is here speaking of a certainty of action and commitment, not of certainty as the subjective state in which anxiety and doubt is absent. He instead writes,
If the forms of verification of modern natural science were the only way in which man could arrive at any certainty, then faith would indeed have to be classified in the real of mere “perhaps” and to be constantly fused with doubt, to be virtually identical with it. But just as a person becomes certain of another’s love without being able to subject it to the methods of scientific experiment, so in the contact between God and man there is a certainty of a quite different kind from the certainty of objectivizing thought. We live faith, not as a hypothesis, but as the certainty on which our life is based.
As Ratzinger indicates, our relationship of faith in God is not simply a notional affirmation of his existence but is instead an unconditional act of trust, irreducibly personal, in Someone. Ratzinger’s comparison to a relationship of love is more than a helpful metaphor, it is an analogy that precisely diagnoses why we err by restricting God to syllogisms:
If two people regard their love merely as a hypothesis that is constantly in need of new verification, they destroy love in that way. It is contradicted in its essence if one tries to make it something one can grasp . . . God, most of all, cannot be objectified as if he were a thing on a lower level than we are.
But in what sense can we say that the interpersonal knowledge of faith or trust is the highest form of human knowledge? First we must note that the Christian tradition has consistently maintained that our current vision by faith is inferior to the fullness of knowledge available in the life to come; yet this knowledge is higher than faith because it is a perfected personal knowledge of love rather than a scientific knowledge. The natural faith between two persons involves more than the operations of the intellect required for mathematical reasoning; such faith is no less an act of love than of intellect, as the entire person is engaged in revealing himself to another and the other in turn trusts his testimony.
As Walsh argues in Politics of the Person as the Politics of Being, “the person is the apex of being” since, in the person, the fundamental tendency of all being toward self-disclosure reaches its climax. Yet, because the interiority of a person is unobservable, knowledge of a person cannot be grasped. Although a person can express himself with words and actions, neither of these can fully translate his hidden inner region of subjectivity; thus, one can only know a person through trust. Knowledge of a person, “the apex of being,” is therefore also the apex knowledge.
Our remarks about natural faith as personal knowledge apply also to the Christian’s supernatural faith, whose object is the Triune God. Human knowledge of God through a personal act of faith imitates the inner-Trinitarian relationship between the Father and Son, in which—according to the analogy of St. Thomas Aquinas—the Word’s procession is akin to the unity between the act of knowing and that which is known:
For it is clear that the more a thing is understood, the more closely is the intellectual conception joined and united to the intelligent agent; since the intellect by the very act of understanding is made one with the object understood. Thus, as the divine intelligence is the very supreme perfection of God, the divine Word is of necessity perfectly one with the source whence He proceeds, without any kind of diversity (ST I.q.27.a.1.ad.2).
From the Christian perspective then, the highest form of knowledge is also the very act of intellect that generates the divine Person of the Son. In the immanent Trinity, personal knowledge is inseparable from the fecundity of divine love and omnipotence.
Humans can adequately regard God only through the interpersonal knowledge made possible by faith, an unconditional trust in the One who makes Himself known in creation, in the human heart, and—according to Christianity—in Jesus of Nazareth, the fullness of God’s self-revelation. If God first addresses us as persons, then our immediate response must not be syllogisms but the personal response known as prayer. Within prayer’s framework of unwavering trust, philosophical and theological reflection elucidate our explicit understanding of God, not so as to justify our love, but so as to love him all the more. Arguments for God’s existence wither when deprived of their orientation in faith, which can only be present when God is no longer approached as a hypothesis, nor even as philosophical first principle, but as the personal mystery whose loving address to us is the same action that calls us into existence.
 Charles Taylor, A Secular Age (Cambridge, MA: Belknap, 2007).
 René Descartes, First Meditation.
 Joseph Cardinal Ratzinger, Introduction to Christianity, transl. J. R. Foster (San Francisco: Ignatius Press, 2004) 45.
 Avery Dulles, The Survival of Dogma (New York: Crossroad, 1987), 140.
 Ibid., 48.
 Newman, Grammar of Assent, Ch 9 §3; cited in Dulles, The Survival of Dogma, 47.
 Pierre Rousselot, S.J., The Eyes of Faith, transl. Joseph Donceel, S.J. (New York: Fordham University Press, 1990), 27.
 Rousselot, Eyes of Faith, 28.
 Ibid., 52.
 Ibid., 50.
 To clarify: I am not saying that the premises of the proofs themselves beg the question by harboring the conclusion at the outset; rather, I simply mean that the basis of the Christian philosopher’s conviction of God’s existence is not reducible to the deductive syllogisms he has arranged.
 See Etienne Gilson, The Spirit of Mediaeval Philosophy (Notre Dame, IN: UNDP, 1991). Gilson argues that although human reason can in principle demonstrate God’s existence apart from faith in divine revelation, it was only after the influence of Christian revelation that philosophical arguments for God’s existence and attributes truly flourished.
 As Henri de Lubac writes: “Knowledge of this kind [the more fundamental intuition of God’s existence] does not make the ‘scientific’ proofs superfluous, but on the contrary it makes them possible, since that is the basic testimony which supports the proofs to which, ultimately, we must always return.” The Discovery of God (Grand Rapids, MI: Eerdmans, 1996) 81.
 David Walsh, Politics of the Person as the Politics of Being (Notre Dame, IN: UNDP, 2016), 10.
 Karl Rahner, Foundations of Christian Faith: An Introduction to the Idea of Christianity (New York: Seabury, 1978).
 Ibid., 44.
 Ibid., 52.
 Even verbal revelation, Rahner notes, is a posteriori insofar as it employs human concepts. Ibid.
 Rahner, Foundations, 52.
 C. Stephen Evans, Natural Signs and Knowledge of God: A New Look at Theistic Arguments (Oxford, UK: Oxford University Press, 2012), 2.
 As Michael Polanyi has shown, we often know more than we can tell and the inability to explain one’s reasons for a conviction do not render it irrational per se. See Michael Polanyi, The Tacit Dimension (Chicago: UCP, 2009).
 De Lubac, The Discovery of God, 83.
 Rahner, Foundations, 60.
 Joseph Cardinal Ratzinger, Pilgrim Fellowship of Faith: The Church as Communion (San Francisco: Ignatius, 2005), 20.
 Ibid. One may notice where the analogy seems to break down: few lovers can be found who genuinely doubt one another’s existence, which is precisely the point at issue with God. In response, I would suggest that, as classical philosophical theism in the Christian tradition has argued, God’s Love is not distinct from his existence (none of the attributes are) since he is perfectly simple, without composition. To trust his existence is to trust his love. Moreover, unlike the love of another human, God’s love is noticeable as a tug within the human spirit and is therefore a much more personal reality to which we must respond.
 Walsh, Politics of the Person, 12.
 Roch A. Kereszty, Jesus Christ: Fundamentals of Christology (New York: St. Paul Society, 2011), 18.