Read John 1:12 again.
John 1:12, "But as many as received Him, to them He gave the right to become children of God, even to those who believe in His name . . .."
When they believed, they received the RIGHT to become children of God. They didn't become children of God yet, but only received the RIGHT to become such. Hmmmmm.... So how do they use that right?
Galatians 3:26-27
For ye are all the children of God by faith in Christ Jesus. For as many of you as have been baptized into Christ have put on Christ.
The right to become a child of God is received at the point of belief, and is used at the point of baptism.