“And because you are sons, God has sent the Spirit of his Son into our hearts, crying, "Abba! Father!” (v6). Paul was concerned that the Galatian believers were losing sight of their new position as adopted children of God, free and beloved, and instead were sliding back into enslavement (v3,8) to religious observances and rituals, whether Jewish or pagan (v10). By faith they were “children of the promise” (v28) and invited to know God personally through Christ; if they tried to return to self-salvation through Law, they would lose their inheritance (v30).
“God sent forth his Son” (v4). By faith in Christ, we become children of the Father, not slaves to the Law.