Why does Paul bring up Abraham in Galatians?
Abraham serves as Paul's Old Testament illustration of faith to refute legalism. Paul shows believers did not receive their glorious position by law-keeping but by believing the Gospel. Abraham believed God, and it was accounted to him for righteousness; he did nothing else—no sacrifices, commandments, or baptism.
Les explains that Galatians is written to refute false doctrine, primarily the inroads of legalism. Legalism appeals to the human concept of having to do something, but Paul's doctrine says we do nothing and instead rest on and believe what God says He has finished on our behalf—the Gospel, the death, burial, and resurrection of Christ. In Galatians 3, Paul points out that they did not come into their glorious position by law-keeping but by faith, by believing the Gospel. So as an illustration of faith, he goes back to the Old Testament and picks up Abraham once again. Verse 6 says Abraham believed God, and it was accounted to him for righteousness. Les asks how much else Abraham did: nothing. He did not bring sacrifice, keep a set of commandments, or get baptized in the Jordan; he did nothing but believe God.