32 Therefore the conclusion reached by faith and argument and thought is that the Lord Jesus both was born and always existed: since if the mind survey the past in search of knowledge concerning the Son, this one fact and nothing else, will be constantly present to the enquirer's perception, that He was born and always existed. As therefore it is a property of God the Father to exist without birth, so also it must belong to the Son to exist always through birth. But birth can declare nothing except that there is a Father and the title Father nothing else except that there is a birth. For neither those names nor the nature of the case, will allow of any intermediate position. For either He was not always a Father, unless there was always also a Son; or if He was always a Father, there was always also a Son; since whatever period of time is denied to the Son, to make His sonship non-eternal, just so much the Father lacks of having been always a Father: so that although He was always God, nevertheless He cannot have been also a Father for the same infinity during which He is God.
Source: On the Trinity (New Advent)