5 Know, then, that the four-and-twenty letters which you possess are symbolic emanations of the three powers that contain the entire number of the elements above. For you are to reckon thus — that the nine mute letters are [the images] of Pater and Aletheia, because they are without voice, that is, of such a nature as cannot be uttered or pronounced. But the semi-vowels represent Logos and Zoe, because they are, as it were, midway between the consonants and the vowels, partaking of the nature of both.
The vowels, again, are representative of Anthropos and Ecclesia, inasmuch as a voice proceeding from Anthropos gave being to them all; for the sound of the voice imparted to them form. Thus, then, Logos and Zoe possess eight [of these letters]; Anthropos and Ecclesia seven; and Pater and Aletheia nine. But since the number allotted to each was unequal, He who existed in the Father came down, having been specially sent by Him from whom He was separated, for the rectification of what had taken place, that the unity of the Pleromas, being endowed with equality, might develop in all that one power which flows from all.
Thus that division which had only seven letters, received the power of eight, and the three sets were rendered alike in point of number, all becoming Ogdoads; which three, when brought together, constitute the number four-and-twenty. The three elements, too (which he declares to exist in conjunction with three powers, and thus form the six from which have flowed the twenty-four letters), being quadrupled by the word of the ineffable Tetrad, give rise to the same number with them; and these elements he maintains to belong to Him who cannot be named.
These, again, were endowed by the three powers with a resemblance to Him who is invisible. And he says that those letters which we call double are the images of the images of these elements; and if these be added to the four-and-twenty letters, by the force of analogy they form the number thirty.
Source: Adversus haereses (New Advent)