Paul is now settling into the middle of the epistle, focusing on defining the gospel against what the Galatians Judaizers have said. Two repeated themes clue us in to Paul’s intent here: calling his readers foolish/dumb/unintelligent (v1 & 3) and “believing what you’d heard” (v2 & 5). In short, believing something other than the true gospel is stupid!
He begins by reminding them of things they already know: the gospel had been preached very clearly (v1), the Spirit had been at work among them (v3), they’d even suffered for the faith in some way (v4). But they’re now trying to grow on their own strength, apart from God!
To combat this, Paul presents Abraham (Genesis 12:1-3) in two lights: he is an example of faith (v6 & 8), and he is the father of all believers (v7 & 9). As an example of faith, we learn from Abraham that faith is from God, not from self. In this sense, faith is a virtue, a grace. As the father of all believers, we learn from Abraham that the Christian identity is beyond nationality. He had received a universal covenant, applying to everyone, not just the family that would become the Jewish people.