Redemption Has Come
MacArthur writes: in God’s timetable, when the exact religious, cultural, and political conditions demanded by his perfect plan were in place, Jesus came into the world. God sent forth his Son. As a father set the time for the ceremony of his son becoming of age and being released from the guardians, managers, and tutors, so God sent his Son at the precise moment to bring all who believe out from under bondage to the law—a truth Jesus repeatedly affirmed (John 5:30, 36, 37; 6:39, 44, 57; 8:16, 18, 42; 12:49; 17:21, 25; 20:21). That the Father sent Jesus into the world teaches his pre-existence as the eternal second member of the Trinity (see also Philippians 2:6-7; Hebrews 1:3–5; Romans 8:3-4).
Wiersbe writes: Paul is careful to point out the dual nature of Jesus Christ (Galatians 4:4), that he is both God and man. As God, Jesus “came forth” (John 16:28); but as man, He was “made of a woman.” The ancient promise said that the Redeemer would be of “the woman’s seed” (Genesis 3:15); and Jesus fulfilled that promise (Isaiah 7:14; Matthew 1:18–25).
Paul has told us WHO came—God’s Son; he has told us WHEN he came and HOW he came. Now he explains WHY he came: “to redeem them that were under the Law” (Galatians 4:5). "Redeem" is the same word Paul used earlier (Galatians 3:13); it means “to set free by paying a price.” A man could purchase a slave in any Roman city, either to keep the slave for himself or to set him free. Jesus came to set us free.
So, to go back into the Law is to undo the very work of Christ on the cross. He did not purchase us to make us slaves, but sons! Under Law, the Jews were mere children, but under grace, the believer is a son of God with an adult standing in God’s family.
Galatians 4:4–5New American Standard Bible: 1995 Update
But when the fullness of the time came, God sent forth His Son, born of a woman, born under the Law,
so that He might redeem those who were under the Law, that we might receive the adoption as sons.