1JO 1:2
We have seen, and bear witness, and show unto you that eternal life, which was with the Father, and was manifested unto us.
1 John 1:2
Jesus always was and always will be the Son of God. He didn’t become the Son when He was born here in Bethlehem. He became a human being, yes. But He was always God’s Son. John knew Jesus as the eternal Son, and he wrote this part of Scripture as a first hand witness of being with, and seeing, the eternal Son of God - as a Man.