Paul compares life before Christ to an heir who is still a child. Though the child owns everything, he lives under guardians until the day his father sets. In the same way, people were held under the basic principles of the world. But when the fullness of time came, God sent His Son, born of a woman, born under the law. Jesus came to redeem those under the law so they could be adopted as God's children.

Because believers are God's children, God sent the Spirit of His Son into their hearts. The Spirit cries out Abba, Father, a warm Aramaic word for father. So the believer is no longer a slave but a son, and if a son, then an heir through God. Paul asks why the Galatians would turn back to weak and poor principles, keeping special days and seasons, after coming to know God.

Paul reminds them of his first visit. He was ill, but they welcomed him like an angel of God. Now he wonders whether he has become their enemy by telling the truth. Like a mother in labor, he longs for Christ to be formed in them. Finally he uses Abraham's two sons as a picture. Hagar's son stands for slavery under the law, and Sarah's son stands for freedom by God's promise. Believers are children of promise, children of the free woman.

Paul develops the guardian image from chapter 3. An heir who is a minor differs little from a slave; though owner of all, he remains under managers until the date set by his father. So too humanity was enslaved under the elementary principles of the world. In the fullness of time, at the moment God appointed, He sent forth His Son, born of a woman, born under the law, to redeem those under the law, so that believers might receive adoption as sons.

The proof of this adoption is the Spirit. God sent the Spirit of His Son into believers' hearts, crying Abba, Father, the intimate address Jesus Himself used in Gethsemane. Therefore each believer is no longer a slave but a son, and an heir through God. Paul is alarmed that the Galatians, now known by God, want to return to weak and beggarly principles, observing days, months, seasons, and years. He fears his labor among them may have been wasted.

Paul's tone becomes personal. He first preached to them because of a bodily ailment, and rather than despise him, they received him as an angel of God, even as Christ Jesus. They would have given him their own eyes. Now the false teachers court them to win their loyalty. Paul, like a mother in labor, suffers again until Christ is formed in them. He closes with an allegory from Genesis. Hagar, the slave woman, corresponds to Mount Sinai and the present Jerusalem in slavery. Sarah, the free woman, corresponds to the Jerusalem above. Like Isaac, believers are children of promise, children not of the slave but of the free woman.