God is truth
"For he, too, has a true [real] soul who has not a great soul; since the essence of body and soul is not the essence of the truth [reality] itself; as is the Trinity, one God, alone, great, true, truthful, the truth. Of whom if we endeavor to think, so far as He Himself permits and grants, let us not think of any touch or embrace in local space, as if of three bodies, or of any compactness of conjunction, as fables tell of three-bodied Geryon; but let whatsoever may occur to the mind, that is of such sort as to be greater in three than in each singly, and less in one than in two, be rejected without any doubt; for so everything corporeal is rejected. But also in spiritual things let nothing changeable that may have occurred to the mind be thought of God. For when we aspire from this depth to that height, it is a step towards no small knowledge, if, before we can know what God is, we can already know what He is not. For certainly He is neither earth nor heaven; nor, as it were, earth and heaven; nor any such thing as we see in the heaven; nor any such thing as we do not see, but which perhaps is in heaven. Neither if you were to magnify in the imagination of your thought the light of the sun as much as you are able, either that it may be greater, or that it may be brighter, a thousand times as much, or times without number; neither is this God. Neither as we think of the pure angels as spirits animating celestial bodies, and changing and dealing with them after the will by which they serve God; not even if all, and there are "thousands of thousands," were brought together into one, and became one; neither is any such thing God. Neither if you were to think of the same spirits as without bodies - a thing indeed most difficult for carnal thought to do. Behold and see, if thou canst, O soul pressed down by the corruptible body, and weighed down by earthly thoughts, many and various; behold and see, if thou canst, that God is truth. For it is written that "God is light;" not in such way as these eyes see, but in such way as the heart sees, when it is said, He is truth [reality].
Ask not what is truth [reality] for immediately the darkness of corporeal images and the clouds of phantasms will put themselves in the way, and will disturb that calm which at the first twinkling shone forth to thee, when I said truth [reality]. See that thou remainest, if thou canst, in that first twinkling with which thou art dazzled, as it were, by a flash, when it is said to thee, Truth [Reality]. But thou canst not; thou wilt glide back into those usual and earthly things. And what weight, pray, is it that will cause thee so to glide back, unless it be the bird-lime of the stains of appetite thou hast contracted, and the errors of thy wandering from the right path?"
St. Augustine, On the Trinity, 8,2