"Arianism is a heresy about the Son, but Athanasius recognizes that it is equally about the Father. He takes Jesus's words quite literally: "He who does not honor the Son does not honor the Father who sent Him" (John 5:23). In dishonoring the Son, the Arians also do injustice to the Father. As Peter Widdicombe points out, "The denial of the eternal fatherhood of God is the first of the heretical doctrines with which Athanasius charges his Arian opponents." They believe, by Athanasius's account, that "God was not eternally Father," and that "God was not always Father, but became so later," and they say both "not eternal Father" and "not eternal Son." Because the Son did not always exist, "God was not always Father". - Leithart, Athanasius, p63.
And while the Scripture does teach that there was a time where the Son became incarnate, there is no teaching that there was a time where the Son wasn't the Son or didn't exist. We know explicitly from Scripture that He is called the Son by the Father in Psalm 2, "I will declare the decree: Yahweh has said to Me, 'You are My Son, Today I have begotten You.'" (v7). Elsewhere we understand "begotten" as eternally proceeding from the Father. This "Today, I have begotten You" is particularly fulfilled in Christ's resurrection (and not his incarnation) when Paul makes his arguments in Acts 13:33. The true Son and not simply David are being referred to long before Jesus is conceived.