On the Christian Faith (De fide)

St. Ambrose questions the heretics and exhibits their answer, which is, that the Son existed, indeed, before all time, yet was not co-eternal with the Father, whereat the Saint shows that they represent the Godhead as changeable, and further, that each Person must be believed to be eternal.

58. Tell me, thou heretic—for the surpassing clemency of the Emperor grants me this indulgence of addressing you for a short space, not that I desire to confer with you, or am greedy to hear your arguments, but because I am willing to exhibit them—tell me, I say, whether there was ever a time when God Almighty was not the Father, and yet was God. “I say nothing about time,” is your answer. Well and subtly objected! For if you bring time into the dispute, you will condemn yourself, seeing that you must acknowledge that there was a time when the Son was not, whereas the Son is the ruler and creator of time. He cannot have begun to exist after His own work. You, therefore, must needs allow Him to be the ruler and maker of His work.

59. “I do not say,” do you answer, that the Son existed not before time; but when I call Him “Son,” I declare that His Father existed before Him, for, as you say, father exists before son. But what means this? You deny that time was before the Son, and yet you will have it that something preceded the existence of the Son— some creature of time,— and you show certain stages of generation intervening, whereby thou dost give us to understand that the generation from the Father was a process in time. For if He began to be a Father, then, in the first instance, He was God, and afterwards He became a Father. How, then, is God unchangeable? For if He was first God, and then the Father, surely He has undergone change by reason of the added and later act of generation.

60. But may God preserve us from this madness; for it was but to confute the impiety of the heretics that we brought in this question. The devout spirit affirms a generation that is not in time, and so declares Father and Son to be co-eternal, and does not maintain that God has ever suffered change.

61. Let Father and Son, therefore, be associated in worship, even as They are associated in Godhead; let not blasphemy put asunder those whom the close bond of generation has joined together. Let us honour the Son, that we may honour the Father also, as it is written in the Gospel. The Son's eternity is the adornment of the Father's majesty. If the Son has not been from everlasting, then the Father has suffered change; but the Son is from all eternity, therefore has the Father never changed, for He is always unchangeable. And thus we see that they who would deny the Son's eternity would teach that the Father is mutable.