You are my witnesses,
says the LORD,
and my servant whom I have chosen: that you may know and believe me, and understand that I am he: before me there was no God formed, neither shall there be after me.
Read Chapter 43
Gaius Marius Victorinus
AD 400
If Christ is Son, Christ is certainly after God. But after God there is nothing comparable to him. Christ is therefore not comparable to God; or if he is not after God, certainly he is with God; for in no way can he be before God; therefore he is consubstantial (homoousion) … substance as substance, especially if it is a homogeneous substance that is realized in two or more individuals, which is said to be identical substance, not similar. - "Against Arius 2.1.1"
Witnesses. The history of the Israelites was sufficient to show who was God. (Calmet)
Thus the establishment of Christianity manifests its truth. (St. Augustine, City of God xxii. 6.)
Septuagint add, "and I am witness. "(Haydock)
Still I would like to ask of our opponents what the words mean that were once uttered by the prophet, “Before me there was no God, and after me there is none.” If the Son is younger than the Father, how can the Father say, “after me there shall be none”? You will not, therefore, also deprive the Only Begotten himself of his substance, will you? Indeed, you must dare to do this or else to accept the one Godhead with the distinct persons of the Father and the Son. - "Homilies on the Gospel of John 4"
“I have glorified you in the world.” By this glory Christ has led the blind people into the light. For the sun of righteousness does not allow us to be children of the night and shadows but rather of day, as the divine apostle says. When Paul says, “They have been gathered,” this in fact has not yet taken place, and he is obviously speaking prophetically, meaning “they will be gathered.” For speaking of things to come as if they had happened is customary in the Scriptures; another example of such is “I have given my back to the rod” and “they divided my clothing among them.” - "Commentary on Isaiah 43.1–13"