37 But now, lest the terms 'creation' and 'establishing' should be an obstacle to belief in the divine birth, these words follow, Before He made the earth, before He made firm the mountains, before all the hills He begot Me. Thus He is begotten before the earth, Who is established before the ages; and not only before the earth, but also before the mountains and hills. And indeed in these expressions, since Wisdom speaks of itself, more is meant than is said.
For all objects which are used to convey the idea of infinity must be of such a kind as to be subsequent in point of time to no single thing and to no class of things. But things existing in time cannot possibly be fitted to indicate eternity; because, from the very fact that they are posterior to other things, they are incapable of suggesting the thought of infinity as a beginning, themselves having their own beginning in time. For what wonder is it, that God should have begotten the Lord Christ before the earth, when the origin of the angels is found to be prior to the creation of the earth?
Or why should He, Who was said to be begotten before the earth, be also declared to be born before the mountains, and not only before the mountains but also before the hills; the hills being mentioned, as an afterthought, after the mountains, and reason requiring that there should be a world before mountains could exist? For such reasons it cannot be supposed that these words were used merely in order that He might be understood to exist prior to hills and mountains and earth, Who surpasses by the eternity of His own infinity things which are themselves prior to earth and mountains and hills.
Source: On the Trinity (New Advent)