Paul's argues that blessing does not come by following the law of Moses. In fact, the rules described in the law bring the threat of God's curse if anyone fails to keep them in any way. Since all fail to keep it perfectly, no one can be justified - made right before God - by the law. Christ took this curse on Himself when He died for our sin on the cross. That's how the blessing of Abraham has come to the Gentiles who believe in Christ by faith.