De trinitate I.1.3

"So then it is difficult to contemplate and have full knowledge of God's substance, which without any change in itself makes things that change, and without any passage of time in itself creates things that exist in time. That is why it is necessary for our minds to be purified before that inexpressible reality can be inexpressibly seen by them; and in order to make us fit and capable of grasping it, we are led along more endurable routes, nurtured on faith as long as we have not yet been endowed with that necessary purification."

St. Augustine says two things here:
1. that the one who seeks to theologize must be pure;
2. that it is difficult to contemplate and have full knowledge of God's substance.

Compare and contrast how St. Gregory the Theologian discusses these two points in his Theological Orations.

ad 1) above:

"In the former Discourse we laid down clearly with respect to the Theologian, both what sort of character he ought to bear, and on what kind of subject he may philosophize, and when, and to what extent. We saw that he ought to be, as far as may be, pure, in order that light may be apprehended by light; and that he ought to consort with serious men, in order that his word be not fruitless through falling on an unfruitful soil; and that the suitable season is when we have a calm within from the whirl of outward things; so as not like madmen to lose our breath; and that the extent to which we may go is that to which we have ourselves advanced, or to which we are advancing." Oration 28.1

ad 2) above:

"It is difficult to conceive God but to define Him in words is an impossibility, as one of the Greek teachers of Divinity (Plato, Tim., 28 E.:"Now that which is created must, as we affirm, of necessity be created by a cause. But the father and maker of all this universe is past finding out; and even if we found him, to tell of him to all men would be impossible.") taught, not unskilfully, as it appears to me; with the intention that he might be thought to have apprehended Him; in that he says it is a hard thing to do; and yet may escape being convicted of ignorance because of the impossibility of giving expression to the apprehension. But in my opinion it is impossible to express Him, and yet more impossible to conceive Him. For that which may be conceived may perhaps be made clear by language, if not fairly well, at any rate imperfectly, to any one who is not quite deprived of his hearing, or slothful of understanding. But to comprehend the whole of so great a Subject as this is quite impossible and impracticable, not merely to the utterly careless and ignorant, but even to those who are highly exalted, and who love God, and in like manner to every created nature; seeing that the darkness of this world and the thick covering of the flesh is an obstacle to the full understanding of the truth. I do not know whether it is the same with the higher natures and purer Intelligences which because of their nearness to God, and because they are illumined with all His Light, may possibly see, if not the whole, at any rate more perfectly and distinctly than we do; some perhaps more, some less than others, in proportion to their rank." Oration 28.4
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Both Augustine and Gregory agree on the need for purity in those who would theologize--a word we need to hear in our day. But I am struck by this distinction between the two: Augustine says that it is difficult to have full knowledge of God's substance; Gregory says that it is impossible.

St. Gregory Palamas said somewhere (?reference?) that Barlaam made two errors with respect to the knowledge of God: first, that we cannot really know God in this life; and second, that those in heaven will know God with respect to his essence.

Now in the west, no distinction exists between God's essence and his attributes/energies. As Aquinas says, “God is all the things that He has...” (On Spiritual creatures article 11). The translator of De trinitate notes, ".... the attributes of God are not accidental qualities that adhere to him, nor are his actions things that he happens to do in time. As the Arians rightly insisted, in God all accidents become substance. God is his attributes, and his attributes are him, and the same goes for his actions." (pg. 45)

If we make no distinction between God's essence and his energies--and let it be noted that this is a position that the West is committed to--then, if we say it is impossible to know God's essence we must be hard-core agnostics (not to say atheists) of a sort. Hence Augustine is committed to say that to know God's substance is difficult.

But if we make the distinction between God's essence and his energies, which the East is committed to, then we can affirm both that we can know God even in this life, by knowing his energies; and that we can never know God in his essence, either in this life or in the age to come.