Natural Knowledge of God as the Primordial Approach to God
1. From Plato and Aristotle to St. Anselm and St. Thomas Aquinas, to Descartes and Leibniz, philosophers have proposed proofs or demonstrations of the existence of God, or, as Thomas Aquinas more modestly puts it, ways through which the intellect is led to the certitude of His existence. All are highly conceptualized and rationalized proofs, specifically philosophic ways of approach. Kant rightly criticized the proof advanced by Descartes (what is called "the ontological argument"), but wrongly claimed to reduce all the ways of demonstration to this particular proof. That was a great error; for the five ways indicated by Thomas Aquinas are completely independent of the ontological argument, and stand firm in spite of all criticism. 
However, it is not these highly conceptualized, rationalized and specifically philosophical ways of approach which I should like to consider at present. When St. Paul affirmed that: "that which is known of God is manifest in them. For God hath manifested it unto them. For the invisible things of Him, from the creation of the world, are clearly seen, being understood by the things that are made; His eternal power also, and divinity." he was thinking not only of scientifically elaborated or specifically philosophical ways of establishing the existence of God. He had in mind also and above all the natural knowledge of the existence of God to which the vision of created things leads the reason of every man, philosopher or not. It is this doubly natural knowledge of God I wish to take up here. It is natural not only in the sense that it belongs to the rational order rather than to the supernatural order of faith, but also in the sense that it is prephilosophic and proceeds by the natural or, so to speak, instinctive manner proper to the first apperceptions of the intellect rationalized elaboration.
Before entering into the sphere of completely formed and articulated knowledge, in particular the sphere of metaphysical knowledge, the human mind is indeed capable of a prephilosophical knowledge which is virtually metaphysical . Therein is found the first, the primordial way of approach through which men become aware of the existence of God.
2. Here everything depends on the natural intuition of being - on the intuition of that act of existing which is the act of every act and the perfection of every perfection, in which all the intelligible structures of reality have their definitive actuation, and which overflows in activity in every being and in the intercommunication of all beings.
Let us rouse ourselves, let us stop living in dreams or in the magic of images and formulas, of words, of signs and practical symbols. Once a man has been awakened to the reality of existence and of his own existence, when he has really perceived that formidable, sometimes elating, sometimes sickening or maddening fact I exist , he is henceforth possessed by the intuition of being and the implications it bears with it.
Precisely speaking, this primordial intuition is both the intuition of my existence and of the existence of things , but first and foremost of the existence of things. When it first takes place, I suddenly realize that a given entity - man, mountain or tree - exists and exercises this sovereign activity to be in its own way, in an independence of me which is total, totally self-assertive and totally implacable. And at the same time I realize that I also exist, but as a thrown back into my loneliness and frailty by this other existence by which things assert themselves and in which I have positively no part, to which I am exactly as naught. And no doubt, in face of my existence others have the same feeling of being frail and threatened. As for me, confronted with others, it is my own existence that I feel to be fragile and menaced, exposed to destruction and death. Thus the primordial intuition of being is the intuition of the solidity and inexorability of existence; and, second, of the death and nothingness to which my existence is liable. And third, in the same flash of intuition, which is but my becoming aware of the intelligible value of being, I realize that this solid and inexorable existence, perceived in anything whatsoever, implies - I do not yet know in what form, perhaps in the things themselves, perhaps separately from them - some absolute, irrefragable existence, completely free from nothingness and death. These three leaps - by which the intellect moves first to actual existence as asserting itself independently of me; and then from this sheer objective existence to my own threatened existence; and finally from my existence spoiled with nothingness to absolute existence - are achieved within the same unique intuition, which philosophers would explain as the intuitive perception of the essentially analogical content of the first concept, the concept of Being. 
Next - this is the second stage - a prompt, spontaneous reasoning, as natural as this intuition (and as a matter of fact more or less involved in it), immediately springs forth as the necessary fruit of such a primordial apperception, and as enforced by and under its light. It is a reasoning without words, which cannot be expressed in articulate fashion without sacrificing its vital concentration and the rapidity with which it takes place. I see first that my being is liable to death; and second, that it is dependent on the totality of nature, on the universal whole of which I am a part. I see that Being-with-nothingness, such as my own being, implies, in order that it should be, Being-without-nothingness - that absolute existence which I confusedly perceived from the beginning as involved in my primordial intuition of existence. But then the universal whole of which I am a part is itself Being-with-nothingness, by the very fact that I am part of it. And from this it follows finally that since this universal whole does not exist by virtue of itself, it must be that Being-without-nothingness exists apart from it. There is another Whole - a separate one - another Being, transcendent and self-sufficient and unknown in itself and activating all beings, which is Being-without-nothingness, that is, self-subsisting Being, Being existing through itself.
Thus the internal dynamism of the intuition of existence, or of the intelligible value of Being, causes me to see that absolute existence or Being-without-nothingness transcends the totality of nature. And there I am, confronted with the existence of God.
3. This is not a new approach to God, it is human reason's eternal way of approaching God. What is new is the manner in which the modern mind has become aware of the simplicity and liberating power, of the natural and in some way intuitive character, of this eternal approach. The science of the ancients was steeped in philosophy. Their scientific imagery was a pseudo-ontological imagery. Consequently, there was a kind of continuum between their knowledge of the physical world and their knowledge of God. This latter knowledge was seen as the summit of the former, a summit which had to be scaled by the multiple paths of the causal connections at work in the sublunar world and the celestial spheres. And the sense of Being, which everywhere and always ruled their thought, was for them an atmosphere too habitual to be regarded as a surprising gift. At the same time, the natural intuition of existence was so strong in them that their proofs of God could take the form of the most conceptualized and the most rationalized scientific demonstrations, and be offered as a skillful unfolding of logical necessities, without losing the inner energy of that intuition. This logical machinery was surreptitiously enlivened by the deep-seated intuition of Being.
We are in quite a different position now. In order to reach physical reality in its own enigmatic way and to conquer the world of phenomena, our science has become a kind of Maya - a Maya which succeeds and makes us masters of nature. But the sense of Being is absent from it. Thus when we come to experience the impact of Being upon our mind, it appears to us as a kind of intellectual revelation, and we become keenly aware both of its awakening and liberating power, and of the fact that it involved a knowledge separate from the sphere of knowledge peculiar to our science. At the same time we realize that the knowledge of God, before being developed in logical and perfectly conceptualized demonstrations, is first and foremost a natural fruit of the intuition of existence, and that it imposes itself upon our mind through the imperative force of this intuition.
In other words, we have become aware of the fact that in its primordial vitality the movement of the human reason in its approach to God is neither a pure intuition (which would be suprahuman), nor the kind of philosophical reasoning of a technical type through which it will be expressed in its achieved form, and which at each of its stages is pregnant with conflicts and with problems to clarify. In its primordial vitality the movement of the human reason in its approach to God is a natural reasoning, that is, intuitive-like or irresistibly maintained in, and vitalized by, the intellectual flash of the intuition of existence. In this natural reasoning it is just this intuition of existence which, seizing in some existing reality Being-with-nothingness, by the same stroke makes the mind grasp the necessity of Being-without-nothingness. And nowhere is there any problem involved, because the illumining power of this intuition takes possession of the mind and obliges it to see, in such a way that the mind proceeds naturally, within a primordial intuitive flash, from imperative certainty to imperative certainty. I believe that from Descartes to Kierkegaard the effort of modern thought - to the extent that it has not completely repudiated metaphysics and if it is cleansed of the irrationalism which has gradually corrupted it - tends to such an awareness of the specific naturalness of man's knowledge of God, definitely more profound than any scientifically developed logical process, and an awareness of the primordial and simple intuitiveness in which this knowledge originates.
4. I have just tried to describe the manner in which this natural prephilosophic knowledge spontaneously proceeds. It involved a reasoning, but a reasoning after the fashion of an intuitive grasp, bathed in the primordial intuition of existence. Let us say that this natural knowledge is a kind of innocent knowledge, a knowledge free of all dialectic. Such a knowledge is rich in certitude, a certitude that is indeed compelling, although it exists in an imperfect logical state. It has not yet crossed the threshold of scientific demonstration, whose certitude is critical and implies that the difficulties inherent in the question have been surmounted through a scrutiny of the rational connections and necessities involved. Such natural knowledge is still in happy ignorance of these difficulties and of all the videtur quod non's : because scientific certitude and the objections to be met - and the replies to the objections - all come into the world together.
It appears, therefore, that the philosophic proofs of the existence of God, let us say the five ways of Thomas Aquinas, are a development and unfolding of this natural knowledge, raised to the level of scientific discussion and scientific certitude. And they normally presuppose this natural knowledge, not with regard to the logical structure of the demonstration, but with regard to the existential condition of the thinking subject. If the preceding observations are true, it would be necessary, before proposing the philosophic proofs, to be assured insofar as possible (by trying, where need be, to aid in such an awakening) that the minds to which one addresses oneself are alive to the primordial intuition of existence, and conscious of the natural knowledge of God involved in this intuition.
One more remark seems to be called for here. I have just used the expression "the philosophical proofs of the existence of God," and I noted above that St. Thomas Aquinas preferred to use the word ways . He had his reasons for this.  These ways are proofs, but the words "proofs" or "demonstration" may be misunderstood. To prove or to demonstrate is, in everyday usage, to render evident that which of itself was not evident. Now, on the one hand, God is not rendered evident by us. He does not receive from us and from our arguments as evidence which He would have lacked. For the existence of God, which is not immediately evident for us , is immediately evident in itself - more evident in itself than the principle of identity, since it is indefinitely more than a predicate contained in the notion of a subject. It is the subject, the divine essence itself (but to know this from immediate evidence, it would be necessary to see God). On the other hand, what our arguments render evident for us is not God Himself, but the testimony of Him contained in his vestiges, His signs or His "mirrors" here below. Our arguments do not give us evidence of the divine existence itself or of the act of existing which is in God and which is God Himself - as if one could have the evidence of His existence without having that of His essence. They give us only evidence of the fact that the divine existence must be affirmed, or of the truth of the attribution of the predicate to the subject in the assertion "God exists." 
In short, what we prove when we prove the existence of God is something which infinitely surpasses us - us and our ideas and our proofs. "To demonstrate the existence of God is not to submit Him to our grapplings, nor to define Him, nor to take possession of Him, nor to handle anything else than ideas that are feeble indeed with regard to such an object, nor to judge anything but our own radical dependence. The procedure by which reason demonstrates that God is places reason itself in an attitude of natural adoration and of intelligent admiration."  And thus the words "proof" and "demonstration," in reference to the existence of God, must be understood (and in fact are so understood spontaneously) with resonances other than in the current usage - in a sense no less strong as to their rational efficacy but more modest in that which concerns us and more reverential in that which concerns the object. On this condition it remains perfectly legitimate to use them. It is just a matter of marking well the differences in station. This being understood, we shall not hesitate to say "proof" or "demonstration" as well as "way," for all these words are synonymous in the sense we have just specified.
As to the very word existence , the existentialist philosophers arbitrarily corrupt its meaning when they say that to exist is "to stand outside of oneself."  But even in its genuine meaning - to stand "outside its causes" or "outside of nothingness" (the etymological sense of the word being " sistere ex , that is to say, to stand or to be posited in itself, from an anterior term on which it depends" ) - the word existence, in order to apply to God, must lose the connotation which thus refers it to created things. It is clear that God does not stand "outside His causes" - as though nothingness preceded God; and that He is not sistens ex - as if He depended on some antecedently existing source. Of itself, however, the notion of existence is in no wise restricted to such a connotation, which in fact refers to the analogue that falls first and immediately under our apprehension; from the outset it overflows all pseudo-definitions carried over from this connotation. Just as the notion of being, the notion of existence is of itself, essentially and from the first, an analogous notion, validly applicable to the uncreated as to the created. No doubt, the word being, in contrast to the word existence, does not need to be purified of accidental vestiges due to etymology. Truth to tell, however, the word existence has been spontaneously purified of them, all by itself, and in any event this does not affect at all the meaning itself of the notion. Those who think that one can say "God is," but not "God exists," maintain for being its essential analogicity but refuse it to existence - the strangest of illusions, since being itself is understood only in relation to existence. To say "God is" and "God exists" is to say exactly the same thing. One speaks the language of simple truth in speaking of the ways through which it is shown that God is, or that He exists.
1 For a discussion of the ontological argument and of the primacy unduly attributed by Kant to this argument, see our Dream of Descartes (New York: Philosophical Library, 1944), Chapter VI.
2 Rom 1:19-20
3 On the concept of Being, see our book Existence and the Existent (New York: Pantheon, 1948), Chapter I.
4 Cf. Les Degrés du Savoir (Paris: Desclée De Brouwer, 1932), pp. 445-446.
5 Cf. De Potentia , q. 7, a .2, ad 1; Summa Theologica , I, 3, 4, ad 2; Les Degrés du Savoir, pp. 837-839.
6 Les Desgrés du Savoir , p. 446.
7 Cf. Existence and the Existent , p. 12, note 3; Michel Sora, Du Dialogue Intérieur (Paris: Gallimard, 1947), p. 30.
8 Etienne Gilson, L'Etre et l'Essence (Paris: Vrin, 1948), p. 249.
Approaches to God, translated by P. O'Reilly (New York: Harper & Brothers, 1954), pp. 1-15.