By giving us the Gospel that has blotted out all the sins of this world, the Lord has turned all of us who believe into God's children.
This is explained by Ephesians 1:4, "Just as He chose us in Him before the foundation of the world". It is in Jesus that God has given us all at once each and every spiritual blessing in Heaven that He has wanted to give us.
Human beings can become God's children only if they receive the remission of their sins by believing in the Gospel and are born again into God's people. This is because we were all born as sinners by nature and the only way for us to become children of God is to believe in Him and what Jesus did for us at the Cross. The main purpose of our faith in the True Gospel is for us to reach salvation from all our sins.