Pace those who think otherwise (like this gentleman, for example), the first object of human knowledge is not God nor any other immaterial thing. St Thomas explains:
It is written (Wisdom 9:16): “The things that are in heaven, who shall search out?” But these substances are said to be in heaven, according to Matthew 18:10, “Their angels in heaven,” etc. Therefore immaterial substances cannot be known by human investigation.
…In the opinion of Plato, immaterial substances are not only understood by us, but are the objects we understand first of all. For Plato taught that immaterial subsisting forms, which he called “Ideas,” are the proper objects of our intellect, and thus first and “per se” understood by us; and, further, that material objects are known by the soul inasmuch as phantasy and sense are mixed up with the mind. Hence the purer the intellect is, so much the more clearly does it perceive the intelligible truth of immaterial things.
But in Aristotle’s opinion, which experience corroborates, our intellect in its present state of life has a natural relationship to the natures of material things; and therefore it can only understand by turning to the phantasms, as we have said above (Question 84, Article 7). Thus it clearly appears that immaterial substances which do not fall under sense and imagination, cannot first and “per se” be known by us, according to the mode of knowledge which experience proves us to have. [ST I q88 a1, emphasis added]
There are a few things worth observing here. First, he’s not saying (of course) that we can know nothing of immaterial substances; he’s only saying that they’re not the first things we know, nor that we just know them in some unmediated way. Second, he says that experience confirms Aristotle’s view. This is important because for a proposition to be true, it must conform to reality: that is, it must consistent with the way that things really are. And I think that he’s exactly right about this, not just in an “if you think about it, you’ll see that he’s right” way, but in a “this is blindingly obvious” way. Because it’s just obvious that we start by getting knowledge by way of our senses.
A little later, Aquinas says that this is not merely true about angels (the immaterial beings in view above), but a fortiori true for God.
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) [ST I q88 a3].
The point, again, is not that we cannot know anything about God. We certainly can, and not merely by resort to Scripture. After all, we’re talking about the man who rather famously proposed Five Ways to show that God exists, and whose Summa Contra Gentiles is a five-volume exercise in natural theology. Rather, his point is that God is not what we know first, and that (as Romans says) we acquire knowledge of Him “by the things that are made.”
Garrigou-Lagrange has some useful things to say that relate to this.
Since its proper object, however, is the essence of the sense world, our intellect can know God and all spiritual beings only by analogy with the sense world, the lowest of intelligible realities, to know which it needs the sense faculties as instruments. In this state of union with body, its manner of knowing the spiritual world is not immediate like that of the angel. So its very definition of the spiritual is negative. Spiritual, it says, is what is immaterial, i. e.: non material. And this negative mode of knowing the spiritual shows clearly that its proper sphere is in the world of sense. [Reality: A Synthesis of Thomistic Thought, ch. 29 (p. 157f)].
This sort of thing is likewise true of the fact that we say that angels (and God) are invisible: they’re not visible. Once again, we resort to negative language because of the limitations imposed by the fact that our knowledge begins with our senses.
Now the gentleman I linked in the first sentence of this post suggests that St Thomas “conveniently” omitted vital context when he quoted Rom. 1:20 (see above). He proposes that verse 19 should have been included:
Because that which is known of God is manifest in them. For God has 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.
The rendering of v. 19 presented by the gentleman is significantly different than this:
that which is known about God is evident within them
Either way, though, I don’t think that it’s warranted to suggest (as he does) that v. 19 demands a Platonist (or maybe presuppositionalist?) view of how we know things, staring with some sort of innate knowledge of God. Because v. 20 actually explains v. 19, so that the point is (as Aquinas affirms) that we learn of God from what we see: from the sense world around us. What we can know of God is manifest because of what we see, not the other way around.