Gal. 4:21- Abraham had two sons, one by a slave (Hagar) and the other by a free woman. (Sarah). But the one by the slave was born according to the impulse of the flesh (law) while the one by the free woman was born as the result of a promise (the seed to come; Jesus).These things are illustrations, for the women represent the two covenants. One is from Mount Sinai (the law) and bears children into slavery. But the Jerusalem above is free, and she is our mother (the covenant of grace, covenant of promise).
In the New Testament the covenant of Grace has included all humanity. Changed from the people (children) of Israel in the OT to all mankind in the NT. Christ came to die for all people, He did set all people free, not the selected few. There is no Jew or Greek, slave or free, male or female; for you are all one in Christ Jesus (Gal. 3:28 HCBS).
Why then was the law given? Does it have any relevance for us now?
Galatians 3:19-26,29 HCSB and the Mirror: It was added because of transgressions until the Seed to whom the promise was made would come. Mirror: “the law was given as a mirror-measuring rod as an intermediary arrangement to make people aware of the extent of their out of sync-ness with the life of their design and at the same time point them to the promise of a redeemer, the Messianic seed”. The law was put into effect through angels by means of a mediator. Now a mediator is not for just one person, but God is one. Is the law therefore contrary to God’s promises? Absolutely not! Mirror: “It emphasises the desperate need for a redeemer to release righteousness in mankind as their life; something the law would certainly be incapable of. Had it been possible for a person to be justified by the law, the promise would be unnecessary”. For if a law had been given that was able to give life, then righteousness would certainly be by the law. But the Scripture has imprisoned everything under sin’s power, so that the promise by faith in Jesus Christ might be given to those who believe. Before this faith came, we were confined under the law, imprisoned until the coming faith was revealed. The law, then, was our guardian until Christ, so that we could be justified by faith. But since that faith has come, we are no longer under a guardian, Mirror: “faith replaced the custodian. Now that faith has come the law is no longer relevant” for you are all sons of God through faith in Christ Jesus. Mirror: “Jesus is the final confirmation of mankind’s redeemed sonship.” 29 Since Christ is the seed of promise, it is only in realising our union with him that we are equally related to Abraham and heirs of the promise. Faith and not flesh relates us to Abraham.
Christ (the seed) himself was born under the law to redeem those under the law, so that we might receive adoption as sons. And because you are sons, God has sent the Spirit of His Son into our hearts, crying, “Abba, Father!” So you are no longer a slave but a son, and if a son, then an heir through God (Gal. 4: 4-7). A mature son is no longer a child who needs the law as a guardian. “You yourselves are our letter, written on our hearts, recognized and read by everyone. It is clear that you are Christ’s letter, produced by us, not written with ink but with the Spirit of the living God—not on stone tablets (like the 10 commandments) but on tablets that are hearts of flesh” (2 Cor. 3:2-3).