From Stromateis 5, 12:

“For both is it a difficult task to discover the Father and Maker of this universe; and having found Him, it is impossible to declare Him to all. For this is by no means capable of expression, like the other subjects of instruction,” says the truth-loving Plato. For he that had heard right well that the all-wise Moses, ascending the mount for holy contemplation, to the summit of intellectual objects, necessarily commands that the whole people do not accompany him. And when the Scripture says, “Moses entered into the thick darkness where God was,” this shows to those capable of understanding, that God is invisible and beyond expression by words. And “the darkness”—which is, in truth, the unbelief and ignorance of the multitude—obstructs the gleam of truth…

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This discourse respecting God is most difficult to handle. For since the first principle of everything is difficult to find out, the absolutely first and oldest principle, which is the cause of all other things being and having been, is difficult to exhibit. For how can that be expressed which is neither genus, nor difference, nor species, nor individual, nor number; nay more, is neither an event, nor that to which an event happens? No one can rightly express Him wholly. For on account of His greatness He is ranked as the All, and is the Father of the universe. Nor are any parts to be predicated of Him. For the One is indivisible; wherefore also it is infinite, not considered with reference to inscrutability, but with reference to its being without dimensions, and not having a limit. And therefore it is without form and name. And if we name it, we do not do so properly, terming it either the One, or the Good, or Mind, or Absolute Being, or Father, or God, or Creator, or Lord. We speak not as supplying His name; but for want, we use good names, in order that the mind may have these as points of support, so as not to err in other respects. For each one by itself does not express God; but all together are indicative of the power of the Omnipotent. For predicates are expressed either from what belongs to things themselves, or from their mutual relation. But none of these are admissible in reference to God. Nor any more is He apprehended by the science of demonstration. For it depends on primary and better known principles. But there is nothing antecedent to the Unbegotten.