How did the Galatians receive their position as believers?
The Galatians received their position as believers not by law keeping but by faith—by believing the Gospel. Paul stresses they did nothing to earn it; they rested on and believed what God says He has finished on their behalf, namely the death, burial, and resurrection of Christ.
Les explains that legalism appeals to the human concept of having to do something, but Paul's doctrine says we do nothing; we rest on and believe what God says He has finished on our behalf. This is the Gospel: the death, burial, and resurrection of Christ. Galatians was written to refute false doctrine, primarily the inroads of legalism. In chapter 3, Paul made the point that the Galatians did not come into their glorious position by law keeping, but by faith, by believing the Gospel. As an illustration of faith, Paul goes back to Abraham: Abraham believed God, and it was accounted to him for righteousness. Abraham did not bring sacrifice, keep a set of commandments, or get baptized in the Jordan; he simply believed God.