5 After this substance had been placed outside of the Pleroma of the Æons, and its mother restored to her proper conjunction, they tell us that Monogenes, acting in accordance with the prudent forethought of the Father, gave origin to another conjugal pair, namely Christ and the Holy Spirit (lest any of the Æons should fall into a calamity similar to that of Sophia), for the purpose of fortifying and strengthening the Pleroma, and who at the same time completed the number of the Æons.
Christ then instructed them as to the nature of their conjunction, and taught those who those who possessed a comprehension of the Unbegotten were sufficient for themselves. He also announced among them what related to the knowledge of the Father—namely, that he cannot be understood or comprehended, nor so much as seen or heard, except in so far as he is known by Monogenes only. And the reason why the rest of the Æons possess perpetual existence is found in that part of the Father's nature which is incomprehensible; but the reason of their origin and formation was situated in that which may be comprehended regarding him, that is, in the Son. Christ, then, who had just been produced, effected these things among them.
Source: Adversus haereses (New Advent)