57 The birth, therefore, does not constitute His nature inferior, for He is in the form of God, as being born of God. And though by their very signification, 'Unbegotten' and 'Begotten' seem to be opposed, yet the Begotten cannot be excluded from the nature of the Unbegotten, for there is none other from whom He could derive His substance. He does not indeed share in the supreme majesty of being unbegotten: but He has received from the Unbegotten God the nature of divinity. Thus faith confesses the eternity of the Only-begotten God, though it can give no meaning to begetting or beginning in His case. His nature forbids us to say that He ever began to be, for His birth lies beyond the beginnings of time. But while we confess Him existent before all ages, we do not hesitate to pronounce Him born in timeless eternity, for we believe His birth, though we know it never had a beginning.
Source: On the Trinity (New Advent)