8 “This is the disciple who testifies of these things, and wrote these things; and we know that his testimony is true. And there are also,” he adds, “many other things which Jesus did, the which, if they should be written every one, I suppose that even the world itself could not contain the books that should be written.” We are not to suppose that in regard to local space the world would be unable to contain them; for how could they be written in it if it could not bear them when written?
But perhaps it is that they could not be comprehended by the capacity of the readers: although, while our faith in certain things themselves remains unharmed, the words we use about them may not unfrequently appear to exceed belief. This will not take place when anything that was obscure or dubious is in course of exposition by the setting forth of its ground and reason, but only when that which is clear of itself is either magnified or extenuated, without any real departure from the pathway of the truth to be intimated; for the words may outrun the thing itself that is indicated only in such a way, that the will of him that speaks, but without any intention to deceive, may be apparent, so that, knowing how far he will be believed, he, orally, either diminishes or magnifies his subject beyond the limit to which credit will be given.
This mode of speaking is called by the Greek name hyperbole, by the masters not only of Greek, but also of Latin literature. And this mode is found not only here, but in several other parts also of the divine literature: as, “They set their mouths against the heavens;” and, “The top of the hair of such as go on in their trespasses;” and many others of the same kind, which are no more wanting in the sacred Scriptures than other tropes or modes of speaking. Of these I might give a more elaborate discussion, were it not that, as the evangelist here terminates his Gospel, I am also compelled to bring my discourse to a close.
Source: Tractates on the Gospel of John (New Advent)