Why do you call Him the only Son of God, seeing that God calls us all His children?

We are children of God not by nature, but only by adoption and by grace, in that God wills to regard us as such (Eph 1:5). But the Lord Jesus who was begotten of the substance of His Father, and is of one essence with Him, is rightly called the only Son of God (John 1:14; Heb 1:2), for there is no other who is God's Son by nature.