7 He is then the Son of God by nature and not by adoption, begotten of the Father. And he that loves Him that begot, loves Him also that is begotten of Him; but he that despises Him that is begotten casts back the insult upon Him who begot. And whenever thou hear of God begetting, sink not down in thought to bodily things, nor think of a corruptible generation, lest you be guilty of impiety. God is a Spirit, His generation is spiritual: for bodies beget bodies, and for the generation of bodies time needs must intervene; but time intervenes not in the generation of the Son from the Father.
And in our case what is begotten is begotten imperfect: but the Son of God was begotten perfect; for what He is now, that is He also from the beginning, begotten without beginning. We are begotten so as to pass from infantile ignorance to a state of reason: your generation, O man, is imperfect, for your growth is progressive. But think not that it is thus in His case, nor impute infirmity to Him who has begotten. For if that which He begot was imperfect, and acquired its perfection in time, you are imputing infirmity to Him who has begotten; if so be, the Father did not bestow from the beginning that which, as you say, time bestowed afterwards.
Source: Catechetical Lectures (New Advent)