Verse 15–18. Paul states that the promise - Abraham’s descendants will be numerous as the stars - predates the law for “four hundred and thirty years” (Gal 3:17, Ex 12:40), and so the law cannot change this promise.
In the book of Genesis, we actually have two promises. One refers to descendants and the other to the land (Gen 12:7; 13:15; 17:7; 24:7). Paul argues in 3:16 that the descendent - the “seed” - does not refer to Isaac, but to Christ - the most important descendent of Abraham (see Matt 1:1), and the descendants do not refer to Israel according to the flesh, but Israel according to the faith - the Church composed of the Jewish and Gentile Christians. They fill the entire land - the world. In Greek, the word “gen” translated as “this land” in Gen 12:7 LXX can also be translated as “the earth” (Gen 1:1 LXX).
At the moment of giving the promise to Abraham, God had already in mind the Gentiles, who would participate in Abraham’s blessing through faith in Jesus Christ. Paul calls this promise given to Abraham “testament - (often translated in English as covenant) - ratified by God” (Gal 3:17). “430 years” later, God also gave the Law, but its purpose was entire different. The inheritance - that the descendants of Abraham would inherit the whole world (see Rom 4:13) - came as a promise and was not of the law.
Verse 19. Why the law then? The reason is “transgressions”. From Abraham to Moses, those “430 years” were already many transgressions. However, the most prominent of all was the stay in Egypt that caused the people to forget their own God. Moses, for example, did not know the God of his fathers (see Ex 3:6) and the main purpose of the plagues was to show the people, who is their God (Ex 6:7).
The law was added, because of transgressions. The law should inform about good and evil, and thus protect from further transgressions. Unfortunately, sin turned out to be so entirely wicked that it used the law to multiply the transgressions (see Rom 5:20; 7:7–13).
Verse 20. The idea that the Law was given to Moses through the angels and not directed by God is also seen in Acts 7:38.53 and Hbr 2:2. Paul stresses concrete difference between the law and the promise. The law was give through the mediators - angels - by the promise was given directly by God - without any mediators.
Verse 21–22. Then, he speaks about a fundamental weakness of the law. The Jewish Rabbi often said: “the more Torah, the more life”. Paul disagrees. Secondary to the promise, the law can only inform, but it cannot transform. The law cannot make alive. If it could, it would indeed replace the promise. If it could, Christ’s coming and His redeeming death “for our sins” (Gal 1:4) would not be necessary. Yet, the entire scripture testifies to the power of sin corrupting the human heart (see Gen 6:5–6; 8:21; Ex 32:9; 33:5; Ps 14:1–3). Thus, all humankind is shut up under sin (Rom 3:9), and the only solution is the promise given to the believing ones through faith in Jesus.
We can distinguish here two periods in the history of humanity and in each person’s personal history:
Faith as the way of justification was not revealed until the time of Christ. The law as schoolmaster should lead to Christ - this argument was probably meant for the Judaizantes, those Jewish Christians missionaries, who preached the other gospel. It also indicates that the role of the law was temporary, only till the coming of Christ. By brining the Jews to Christ - to faith in Christ - the law fulfilled its role. From now on, Christ takes over. Now, coming back to the law would mean to go back to the period of slavery, to put ourselves again in the hands of schoolmaster.
Verse 26. Up to now, Paul was talking about the Gentiles as being “the children of Abraham” by having the same faith like Abraham (Gal 3:7–9). Now, Paul speaks about the Gentiles as being “sons of God” (see John 1:12). Again this privilege is not confined to the people of Israel (see Ps 82:6) and rooted in circumcision, as a sign of the covenant (see Gen 17:10–11). This privilege is rooted in the sacrament of baptism, administered not just to men - like circumcision - but to all. The consequence of baptism are described in Rom 6:1–14).
27–29. Here, Paul speaks about its transforming power:
Verses 4:1–2. However, there is still one more problem, namely legal immaturity of the heir. In the ancient world, the position of a child before reaching the age of legal maturity was not different from that of a slave. Although, that child was a future heir, he could not yet use freely the inheritance to be given to him by his father and was under schoolmaster. Only after reaching the age of maturity, he was free to use the inheritance (see Luke 15:11–13). We are not sure whether the same was true in Judaism in Palestine (remember that Paul was a Jew from diaspora).
Verse 4:3. The image of an immature child is applied to the believers in Christ. What could that period of immaturity mean? In the case of Gentile Christians, a life before faith under the influence of idols, pagan gods. In the case of the Jewish Christians, probably a life under the law given by the angels or also a life of idolatry exposed by the prophets (see 1 Kings 18:21; Jer 44:15–19).
Paul calls that kind of former life, a life “under the elements of the world”, which refers to the ancient world view that saw powerful forces - including angels - as controlling the world and human life (see 1 Cor 2:8; Eph 6:12; Col 2:8.18).
This period of immaturity, like the period of slavery, ends with the coming of Christ, with our coming to faith in Him.