Professor Gilson, in his Introduction d l'etude de Saint Augustin, remarks that in the thought of St. Augustine there is really one long proof of God's existence, a proof which consists of various stages. Thus from the stage of initial doubt and its refutation through the Si fallor, sum, which is a kind of methodical preliminary to the search for truth, assuring the mind of the attainability of truth, the souls proceeds to consider the world of sense. In this world, however, it does not discover the truth which it seeks and so it turns inwards, where, after considering its own fallibility and changeableness, it discovers immutable truth which transcends the soul and does not depend on the soul. It is thus led to the apprehension of God as the Ground of all truth.
The picture of Augustine's total proof of the existence of God given by M. Gilson is doubtless representative of the Saint's mind and it has the great advantage not only of bringing into prominence the proof from thought, from the eternal truths, but also of linking up the 'proof' with the soul's search for God as the source of happiness, as objective beatitude, in such a way that the proof does not remain a mere academic and theoretic string or chain of syllogisms. This picture is confirmed by a passage such as that contained in Augustine's two hundred and forty-first sermon, where the Saint depicts the human soul questioning the things of sense and hearing them confess that the beauty of the visible world, of mutable things, is the creation and reflection of immutable Beauty, after which the soul proceeds inwards, discovers itself and realises the superiority of the soul to the body. 'Men saw these two things, pondered them, investigated both of them, and found that each is mutable in man.' The mind, therefore, finding both body and soul to be mutable goes in search of what is immutable. 'And thus they arrived at a knowledge of God the Creator by means of the things which He created.' St. Augustine, then, in no way denies what we call a 'natural' or 'rational' knowledge of God; but this rational knowledge of God is viewed in close connection with the soul's search for beatifying Truth and is seen as itself a kind of self-revelation of God to the soul, a revelation which is completed in the full revelation through Christ and confirmed in the Christian life of prayer. Augustine would thus make no sharp dichotomy between the spheres of natural and revealed theology, not because he failed to see the distinction between reason and faith, but rather because he viewed the soul's cognition of God in close connection with its spiritual search for God as the one Object and Source of beatitude. When Harnack reproaches Augustine with not having made clear the relation of faith to science, he fails to realise that the Saint is primarily concerned with the spiritual experience of God and that in his eyes faith and reason each have their part to play in an experience which is an organic unity.
Frederick Coplestone, A History of Philosophy, Vol. 2: Medieval Philosophy From Augustine to Duns Scotus, pp. 70-1