There was strong division in early Christianity over when Jesus became the Son of God. Some believed Jesus became the Son of God at his baptism, some believed it was at his resurrection, others believed he had always been God's son.
The earliest gospel, Mark, never mentions a virgin birth, nor do the epistles of Paul, which predate the gospels. You find the virgin birth in Matthew and Luke, which came into being 50 to 60 years after Jesus died, but you don't find it mentioned in Paul's works 20 years before these. Over time it appears Jesus may have evolved into this virgin birthed son of God, who existed as God's only son before his birth.
Paul seems to indicate Jesus became the Son of God at his resurrection.
Acts 13:33-34: "And we declare to you glad tidings—that promise which was made to the fathers. God has fulfilled this for us their children, in that He has raised up Jesus; as it is written in the second Psalm: ‘You are My Son, Today I have begotten You
.’ And that He raised Him from the dead, no more to return to corruption, He has spoken thus: ‘I will give you the sure mercies of David."
Scholar Bart D. Ehrman shows how early texts of the Gospel of Luke indicate Jesus became God's son at his baptism.
One of the most intriguing antiadoptionist variants among our manuscripts occurs just where one might expect it, in an account of Jesus's baptism by John, the point at which many adoptionists insisted Jesus had been chosen by God to be his adopted son. In Luke's Gospel, as in Mark, when Jesus is baptized, the heavens open up, the Spirit descends upon Jesus in the form of a dove, and a voice comes from heaven. But the manuscripts of Luke's Gospel are divided concerning what exactly the voice said. According to most of our manuscripts, it spoke the same words one finds in Mark's account: "You are my beloved Son in whom I am well pleased" (Mark 1:11; Luke 3:23). In one early Greek manuscript and several Latin ones, however, the voice says something strikingly different: "You are my Son, today I have begotten you." Today I have begotten you! Doesn't that suggest that his day of baptism is the day on which Jesus has become the Son of God? Couldn't this text be used by an adoptionist Christian to make the point that Jesus became the Son of God at this time? As this is such an interesting variant, we might do well to give it a more extended consideration, as a further illustration of the complexities of the problems that textual critics face. - Misquoting Jesus, pages 158-159, Bart D. Ehrman, Professor of New Testament Studies, North Carolina, Chapel Hill.
It wasn't until the proto-orthodox got the stronghold that such rivaling opinions were silenced. In the dark ages, when common men didn't have the scriptures of the New Testament, people agreed, because they accepted what was taught by the Catholic Church; but once people got the scriptures for themselves, and had access to ancient texts, etc., Christianity began to divide on issues.
Point of this thread? Jesus was probably born naturally, and over time myths grew about his virgin birth, and how he was the only begotten Son of God before the world was. Paul epistles and the earliest gospel Mark never mention a virgin birth, but by the time John came along, a good 80 years after Jesus died, Jesus is God Almighty, who was the only begotten Son of God even before Abraham.