Verse 4:4. “The fullness of time”. Like in the life of a child comes the age of maturity and then the Father gives him his inheritance, so is in the case of the world (Gal 4:4).
Christ is sent and then born of a woman, which indicates that Christ existed before He was born of a woman (see Rom 1:3–4; 1 Cor 8:6; 2 Cor 8:9; Phil 2:6; Col 1:15–18).
“Born under the law”. As any Jew, Jesus submitted to the requirements of the law (see Luke 2:22–24.39). In is interesting to notice Paul’s idea regarding Christ’s incarnation:
Thus, in Christ’s incarnation there is this threefold purpose, threefold liberation of human person.
Regarding the Law, we should constantly bear in mind its temporary function. There was no law from Abraham to Moses, and the coming of Christ ends the law (Rom 10:4). The period of the law was from Moses to Christ only. From this perspective it is easy to understand the statement of Acts 21:21. Since the time of the law passed away, then there is no need for circumcision nor for other customs of the law. What has replaced the law? Faith acting through love (Gal 5:6).
4:5–7. “Adoption of sons”. This is legal term. A man legally acknowledges somebody as his child and an adoption in a court needed witnesses. Paul applies this concept to all Christians, living before in the stage of immaturity. We have received the adoption, and the Holy Spirit is our witness. He confirms our adoption by enabling us to call God, “Abba, Father”. It is important to notice that the Holy Spirit is called “the Spirit of his [Father’s] Son”. Thus, at the moment of our baptism, we have received the same Spirit that Christ has (see Mark 1:9–11).
The consequence of this experience is similarly presented to that of Gal 3:29. The change is from a slave to a son, with a right to inheritance, participation in God’s promise given to Abraham.
The Galatians before the Gospel was preached to them by Paul were like other Gentiles worshipping idols. By believing the Gospel, they have become adopted children of God. How come, they want to submit themselves back to those “elements” (the same word like in Gal 4:3 - there, it were “the elements of the world”; here, it is “the weak and poor element”).
Now, we could sketch a possible conversion path for a Gentile:
But, there is no way to go from Christianity to Judaism. That would return to slavery not a conversion.
4:10–11. The problem is that the Galatians have already embraced some of the religious practices of Judaism (see John 7:37; Is 1:13; Lev 25:2–3.10). What seems to be lacking is only circumcision of their male members. But, that would mean the failure for Paul’s missionary efforts.
4:12–18. Aside from logical arguments, Paul also tries to win the back by using emotional argument. The apostle reminds them the circumstances of his first visit that gave him the occasion to preach the Gospel. Yet, how different the situation now. Before, deep affection; now enmity and probable rejection of him and the Gospel he preached.
The freedom of the Gospel preached by Paul turned him from “as an angel of God, as Christ Jesus” to their “enemy”. How could that be? Behind this change are certain people, whose main purpose was to cut that joyful relationship that was built between Paul and the Galatians.
Yet, not only the Judaizantes should be blamed, but the Galatians. When, he was with them, they should him zealous affection. When, he left and other came in, the Galatians shift their affection immediately to those Jewish missionaries. That what surprised Paul (Gal 1:6).
4:19–20. The phrase “travail in birth” refers to the effect of preaching the Gospel, which is compared to giving birth (see 1 Cor 4:15; John 3:3). By means of this letter, Paul tries to preach the true Gospel again. It indicates that Paul is aware that his first attempt has somehow failed. Moreover, Paul even lost has confidence in a successful outcome of his efforts - “I stand in doubt of you”.
4:21–27. In this part, we have an example of classical method of interpreting the Bible, captured in the famous statement of Saint Augustine: “the New Testament lies hidden in the Old, and the Old is unveiled in the New”.
Hagar, the slave woman, represents:
(1) Sinai - the law - slavery;
(2) Earthy Jerusalem;
(3) the Jews born according to the flesh - a clear reference to Ismael, the son of Abraham and Hagar.
Sarah, the free woman, represents:
(1) heavenly Jerusalem;
(2) freedom;
(3) the Jewish and the Gentile Christians born according to the promise, namely believing in Christ - a clear reference to Isaac, the promised child of Abraham and Sarah.
A slave woman can only give birth to a slave. It is the free woman that can give birth to a free child. Paul sees the covenant at Sinai (Ex 24:7–8) as giving birth to slaves - put under the law (Gal 4:24). The second covenant is connected with Jerusalem above (Heb 12:22; Rev 21:2.10–27). This covenant can only have one meaning, namely that in Christ (see Matt 26:28). Here, there is no law but trust in God’s promise.
This Jerusalem above - “our mother” - refers to the Church, free from the law. She gave birth to many children, much more than the children of the earthly Jerusalem (Is 54:1), but it is a spiritual birth (see John 3:3–7) through the sacrament of baptism. This spiritual birth gives also special citizenship, also above, supernatural, namely heavenly (Phil 3:20).
4:28. This allegory is applied now to the Galatians. They should see themselves as Isaac, children of the promise, not born according to the flesh - refers to Ismael and the slavery of the law, but according to the spirit - a spiritual birth through God’s grace in the sacrament of baptism.
4:29. The reference to persecution could refer to those Judaizantes - the Jewish Christian missionaries - who tried to pervert the Gospel of Christ. They would represent Ismael, and all true believers would represent Isaac. Paul refers here to Gen 21:9, which speaks about Ismael “playing” or “laughing”. Both words are at the root of Isaac’s name (see Gen 17:17; 18:12). In English, that seems rather innocence, but the meaning indicates that Ismael was ‘taking Isaac’s place’. That is the reason for Sarah’s strong demand (Gen 21:10): a child of a slave could not inherit what belonged to a free child.
4:30–31. The quote from Gen 21:10 ends Paul’s argument. Paul hopes that their understand the spiritual meaning behind that quote. It basically summarizes the whole argument: If one submits himself to the law, one loses the promise of inheritance.