Baptism is the rite of adoption. We are all children of God by Creation, and Christ alone is Gods Son by natural and eternal generation. Galatians 4:4-5; Romans 8:15-16 state, “Ye received not the spirit of bondage again unto fear; but ye received the spirit of adoption whereby we cry, Abba Father. The Spirit Himself beareth witness with our spirits, that we are the children of God; and if children, then heirs; heirs of God and joint heirs with Christ; if so be that we suffer with Him, that we may be also glorified with Him. Paul infers this through the rite of Baptism when he states in Galatians 3:26-27, “Ye are all sons of God through faith in Jesus Christ. For as many of you as were baptized into Christ did put on Christ.”
So the rite of Baptism is a sign to the world of our regeneration through a new birth, and whereby we are engrafted into the Church. Through this act the Holy Spirit visibly signs and seals the promise of the forgiveness of sin and our adoption as sons of God. John says I was told that I would recognize the Son. Upon whom the Spirit would descend in baptism that would be the Messiah. So the Holy Spirit through Baptism declares that we have received Him and are therefore now, also, the sons of God.
From the Articles on Religion