Adoption as Sons / Heirs
Those who have faith in Christ have grown up. They are no longer children without inheritance and freedom; they are sons.
Wiersbe writes: the entire Trinity is involved in our spiritual experience: God the Father sent the Son to die for us, and God the Son sent his Spirit to live in us. The contrast here is not between immature children and adult sons, but between servants and sons. Like the Prodigal Son, the Galatians wanted their Father to accept them as servants, when they really were sons (Luke 15:18–19).
The son has the same nature as the father, but the servant does not. When we trust Christ, the Holy Spirit comes to live within us; and this means we are “partakers of the divine nature” (2 Peter 1:4). The Law could never give a person God’s nature within.
The son has a father, while the servant has a master. No servant could ever say “Father” to his master. When the sinner trusts Christ, he receives the Holy Spirit within, and the Spirit tells him that he is a child of the Father (Romans 8:15–16).
The son obeys out of love, while the servant obeys out of fear. The Spirit works in the heart of the believer to quicken and increase his love for God. “The fruit of the Spirit is love” (Galatians 5:22). “The love of God is shed abroad in our hearts by the Holy [Spirit]” (Romans 5:5).
The son is rich, while the servant is poor. We are both “sons and heirs.” And since we are adopted—placed as adult sons in the family—we may begin drawing on our inheritance right now. God has made available to us the riches of His grace (Ephesians 1:7; 2:7), the riches of His glory (Philippians 4:19), the riches of His goodness (Rom. 2:4), and the riches of His wisdom (Romans 11:33ff)—and all of the riches of God are found in Christ (Colossians 1:19; 2:3).
The son has a future, while the servant does not. While many kind masters did provide for their slaves in old age, it was not required of them. The father always provides for the son (2 Corinthians 12:14).
Galatians 4:6–7New American Standard Bible: 1995 Update
Because you are sons, God has sent forth the Spirit of His Son into our hearts, crying, “Abba! Father!”
Therefore you are no longer a slave, but a son; and if a son, then an heir through God.