Adoption is an act of God's free grace, whereby we are received into the number, and have a right to all the privileges of the Sons of God.
“But as many as received Him (Jesus Christ), to them He gave the right to become children of God, to those who believe in His name: who were born, not of blood, nor of the will of the flesh, nor of the will of man, but of God.” ~ John 1:12-13 (NKJV)
“The Spirit Himself testifies with our spirit that we are children of God, and if children, heirs also, heirs of God and fellow heirs with Christ, if indeed we suffer with Him so that we may also be glorified with Him.” ~ Romans 8:16-17 (NASB)
“He (God) predestined us to adoption as sons through Jesus Christ to Himself, according to the kind intention of His will.” ~ Ephesians 1:5 (NASB)
“See how great a love the Father has bestowed on us, that we would be called children of God; and such we are.” ~ 1 John 3:1 (NASB)