Continuing the subject of justification by faith in Christ or by faith plus works of the Mosaic Law, Paul demonstrates that God's promise to Abraham was made prior to and independently of the Mosaic Law, which could not in any way change the terms of the promise, but was given to Israel to train it in the way of God, before receiving the blessings of the promise.
In this passage, Paul insists that, even in common life among men, a solemn promise or contract, duly ratified by signature and registration, cannot be changed, added to or withdrawn.
Now then, the promise was made to Abraham in favour of his Descendant (Christ); the Mosaic Law, given 430 years later, could not abrogate or modify the terms of the promise. Note that the word "seed" (Genesis 12:7) in Hebrew can be singular or plural, referring to one or more people (as in English) and here Paul regards it as referring to a Person, Jesus Christ, through whom all nations would be blessed by faith and not by the Mosaic Law.
The Gospel announces the fulfilment of the promise made to Abraham, for the great blessing of salvation by God's grace is offered to all nations without distinction, by faith in Jesus Christ, Son of God and the Seed of Abraham (v. 26).
Still dealing with the times before the Gospel, Paul says that "as long as the heir is a child…” (extending the figure of 3:24-25).
The Jews were under the Law and the Gentiles under their conscience, which pointed to the moral principles of the Law. The ceremonies and rites of the Law were "the elements of the world" (v.3) - the lessons of "primary" and all was preparing the world (Jews and Gentiles) for the "fullness of time", the spiritual maturity.
Before being Christians, they served false gods (idols), for they did not know the true God. Now, having been made heirs and sons of the true God through faith in Christ, they were returning to the rites and ceremonies ("weak and beggarly elements") of the Law - thus making themselves "children" again! Were they really believers in Christ?
Or had they not yet understood the Gospel which Paul had preached to them?
15 Brethren, I speak in the manner of men: Though it is only a man's covenant, yet if it is confirmed, no one annuls or adds to it.
16 Now to Abraham and his Seed were the promises made. He does not say, "And to seeds," as of many, but as of one, "AND TO YOUR SEED," who is Christ.
17 And this I say, that the law, which was four hundred and thirty years later, cannot annul the covenant that was confirmed before by God in Christ, that it should make the promise of no effect.
18 For if the inheritance is of the law, it is no longer of promise; but God gave it to Abraham by promise.
19 What purpose then does the law serve? It was added because of transgressions, till the Seed should come to whom the promise was made; and it was appointed through angels by the hand of a mediator.
20 Now a mediator does not mediate for one only, but God is one.
21 Is the law then against the promises of God? Certainly not! For if there had been a law given which could have given life, truly righteousness would have been by the law.
22 But the Scripture has confined all under sin, that the promise by faith in Jesus Christ might be given to those who believe.
23 But before faith came, we were kept under guard by the law, kept for the faith which would afterward be revealed.
24 Therefore the law was our tutor to bring us to Christ, that we might be justified by faith.
25 But after faith has come, we are no longer under a tutor. 26 For you are all sons of God through faith in Christ Jesus. 27 For as many of you as were baptized into Christ have put on Christ. 28 There is neither Jew nor Greek, there is neither slave nor free, there is neither male nor female; for you are all one in Christ Jesus. 29 And if you are Christ's, then you are Abraham's seed, and heirs according to the promise.
Galatians chapter 3 verses 15 to 29
1 Now I say that the heir, as long as he is a child, does not differ at all from a slave, though he is master of all,
2 but is under guardians and stewards until the time appointed by the father.
3 Even so we, when we were children, were in bondage under the elements of the world.
4 But when the fullness of the time had come, God sent forth His Son, born of a woman, born under the law,
5 to redeem those who were under the law, that we might receive the adoption as sons.
6 And because you are sons, God has sent forth the Spirit of His Son into your hearts, crying out, "Abba, Father!"
7 Therefore you are no longer a slave but a son, and if a son, then an heir of God through Christ.
8 But then, indeed, when you did not know God, you served those which by nature are not gods.
9 But now after you have known God, or rather are known by God, how is it that you turn again to the weak and beggarly elements, to which you desire again to be in bondage?
10 You observe days and months and seasons and years.
11 I am afraid for you, lest I have labored for you in vain.
Galatians chapter 4 verses 1 to 11