There is much that one could say about knowing and comprehending God. What I write below is just an initial reply to some arguments posed by Trinitarians regarding God's cognoscibility.
(1) I think that Augustine of Hippo was wrong when he said that it's easier to know what God is not, as opposed to knowing what or who God is. This type of theology is clearly apophatic: it is an insidious form of "Christian" agnosticism that fails to treat the divine revelation found in the Bible justly. Matthew 11:27 proclaims that "No one knows the Son except the Father, and no one knows the Father except the Son and those to whom the Son chooses to reveal him" (NIV).
Additionally, John 1:18 informs us that the Son of God came and "explained" or "revealed" (EXHGHSATO) the Father. These verses indicate that we do in fact know God by means of the Son. If we do not know what God is, this would invalidate the fact that God has been revealed through His Son. Compare John 17:3; 1 John 5:20.
(2) Some persons may assert that God can be known in his economy, but not immanently (in his essence). Systematic theologian Gordon Kaufman insists that the distinction between God's "essence" and his "economy" is a misleading one. God is who he has revealed himself to be in his economy (OIKONOMIA). If God has been revealed in a certain way through his Son, then we can expect that the revelation conforms to God's essence. For example, if God has revealed that he is triune by means of his Son, then it would seem that God is triune in ESSENTIA. On the other hand, if God has not revealed himself to be three in one through the Son, then the divine one is likely not triune with respect to his ESSENTIA.
(3) Lastly, while I would agree that God is incomprehensible--we can never understand or know everything there is to know about God--I would also contend that there is no biblical evidence to suggest that the Supreme God is triune or that his essence is unknowable. The point can be illustrated by a story which the late historian Jaroslav Pelikan (who was a Trinitarian) tells. He relates a narrative about rabbi Herbanus who debated with the sixth century Orthodox Bishop Gregentius.
(a) Herbanus reasoned that "God is one, and not two or three, as you [Gregentius] say." He added that the Old Testament refers to "sons of God," who are not of the same OUSIA as the Father (i.e. they are not from the OUSIA of the Father, yet they are "sons"). Gregentius responded by quoting Genesis 1:26, where God utters the fateful words, "Let us make man in our image." Herbanus applied this passage to the angels but Gregentius disagreed. To this day the problem has not been resolved from an exegetical perspective, and it will undoubtedly serve as a perennial cause for debate in this AIWN. Be that as it may, certain Trinitarians have written essays demonstrating that neither the Old Testament nor the New Testament explicitly states or even implies that the Father, Son, and Holy Spirit are one. Alan J. Hauser of Appalachian State University concluded that the Trinity doctrine is a "later development" in Christian history. He does not reject the doctrine for this reason, but Hauser's candor about the Biblical testimony is refreshing.
See The Christian Tradition (Vol. II:204-206) by Jaroslav Pelikan.
Early Christian Thinkers By Foster, Paul (EDT)