Objection 1. It would seem that God is the first object known by the human mind. For that object in which all others are known, and by which we judge others, is the first thing known to us; as light is to the eye, and first principles to the intellect. But we know all things in the light of the first truth, and thereby judge of all things, as Augustine says (De Trin. xii, 2; De Vera Relig. xxxi; [Confess. xii, 25). Therefore God is the first object known to us.
Objection 2. Further, whatever causes a thing to be such is more so. But God is the cause of all our knowledge; for He is "the true light which enlighteneth every man that cometh into this world" (John 1:9). Therefore God is our first and most known object.
Objection 3. Further, what is first known in the image is the exemplar to which it is made. But in our mind is the image of God, as Augustine says (De Trin. xii, 4,7). Therefore God is the first object known to our mind.
On the contrary, "No man hath seen God at any time" (John 1:18).
I answer that, Since the human intellect in the present state of life cannot understand even immaterial created substances (1), much less can it understand the essence of the uncreated substance. Hence it must be said simply that God is not the first object of our knowledge. Rather do we know God through creatures, according to the Apostle (Romans 1:20), "the invisible things of God are clearly seen, being understood by the things that are made": while the first object of our knowledge in this life is the "quiddity of a material thing," which is the proper object of our intellect, as appears above in many passages (84, 7; 85, 8; 87, 2, ad 2)
Reply to Objection 1. We see and judge of all things in the light of the first truth, forasmuch as the light itself of our mind, whether natural or gratuitous, is nothing else than the impression of the first truth upon it, as stated above (Question 12, Article 2). Hence, as the light itself of our intellect is not the object it understands, much less can it be said that God is the first object known by our intellect.
Reply to Objection 2. The axiom, "Whatever causes a thing to be such is more so," must be understood of things belonging to one and the same order, as explained above (81, 2, ad 3). Other things than God are known because of God; not as if He were the first known object, but because He is the first cause of our faculty of knowledge.
Reply to Objection 3. If there existed in our souls a perfect image of God, as the Son is the perfect image of the Father, our mind would know God at once. But the image in our mind is imperfect; hence the argument does not prove.
Source: Summa Theologica (New Advent)