In Galatians 3:6, Paul says that Abraham had faith in God in Genesis 15:6, and it was “credited to him as righteousness.” Abraham’s right standing with God was because of his faith, not because of circumcision. This promise was not just for Jews, however, but Gentiles as well. God said to Abraham, “Through you shall all the nations be blessed,” meaning not just the Jewish nation, but the whole world (Gen. 12:3). So, all people will be apart of Abraham’s family that is defined by faith. Now, it might be argued that faith is accompanied with circumcision, because Abraham did eventually receive circumcision in Genesis 17. But that was two chapters after God had said Abraham was righteous by faith. Faith was mentioned first. Does that addition of circumcision two chapters later have to continue to be followed, though? Paul thinks not, for all who follow those outward “works” of the Mosaic Law, circumcision being the prime example, are under a “curse” (Deut. 27:26). Israel had disobeyed the Covenant and were cursed with exile, so now all who should receive circumcision will go back into that cursed Covenant. Paul points out that righteousness is not equated with circumcision, rather with faith. For Habakkuk 2:4 says “the one who is righteous by faith will live.”
Paul gives an analogy between the Covenant with Abraham and a human covenant. When men make covenants between themselves, neither party can “annul” or “amend” it. If that is the case, how much more so with God, who made a Covenant with Abraham? God gave certain promises to Abrahahm and to his “descendant” not to many descendants, but one singular “descendant” (Genesis 13:15 or 17:8). That descendant is most perfectly fulfilled in Christ. So, God made a promise to Abraham back in Genesis (~1700BC) which was to be fulfilled ~1700 years later in Christ (~30AD). After God made that promise to Abraham, however, God gave the “law,” which was four hundred and thirty years after He gave the promise (~1400BC, Gal. 3:17). Just as with human covenants, once a covenant is ratified, it cannot be annulled or amended. That is the reason Paul compares human covenants to the Abrahamic Covenant. Paul says that man does not become a child of Abraham by the law, but by the Abrahamic Covenant. That Covenant was fulfilled in Christ, and now all men have to be “in Christ” to be Abraham’s and God’s children (Gal. 3:26).
Some might get the impression that Paul is saying that Christians do not have to follow morality since the law has been done away with. This is because Paul says things in Galatians chapter three that seem that morality is not necessary. He says that the law was “added for transgressions” and did not bring “righteousness” (Gal. 3:19,21). The law was the “disciplinarian” who told the Jewish people what was sin, but now that disciplinarian has passed away (Gal. 3:24-25). Instead it is by faith. Paul clarifies what that faith entails, however, by going into various moral teachings in Gal. 5 and 6. He says in Gal. 5:6 that faith is “working through love.” Christians who are “in” the Messiah and live by faith, which necessitates that they “serve one another through love” (Gal. 5:13). The whole law, which has been done away with, is summed up and fulfilled in the New Covenant “love” (Gal. 5:14) which has been poured into our hearts (Romans 5:5). That “love” is the presence of the Holy Spirit, who is love itself (1 John 4:16), and Paul says Christians must “live” by Him (Gal. 5:16). They must avoid works of the “flesh,” that is, sins like “idolatry” or “envy” (Gal. 5:20-21). Christians should push one another to grow in holiness by correcting with gentleness Christians who sin (Gal. 6:1). Paul finally gives the analogy of a sower and a field. If the sower sows good works, he will “reap eternal life from the spirit” (Gal. 6:8). If the sower sows bad works -sins- then he will “reap corruption from the flesh” (Gal. 6:8). So, Christians need to do “good to all” (Gal. 6:10).