"[...]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."

"[...]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.
It remains that we understand, then, the Unknown, by divine grace, and by the  word alone that proceeds from Him; as Luke in the Acts of the Apostles relates that Paul said, “Men of Athens, I perceive that in all things ye are too superstitious. For in walking about, and beholding the objects of your worship, I found an altar on which was inscribed, To the Unknown God. Whom therefore ye ignorantly worship, Him declare I unto you.”" (Clement of Alexandria, Str. 5.7)