Paul reminds his Jewish readers that Abraham was forgiven on the basis of believing what God told him, not by keeping the Old Testament Law (Genesis 15:6). How could Abraham have kept laws that didn’t even exist yet? If a person were to earn righteousness by keeping the law, they would be required to keep all of it all the time. That’s 613 rules! Although the law was great at letting people know they needed forgiveness, it never brought about forgiveness. You either depend entirely on the law and be perfect or trust Jesus and let him pay for all the times you have failed to keep the law. Since keeping the law perfectly is impossible, the right choice is obvious.
The mixing of trusting Jesus but requiring some extra stuff is tempting. We have received the priceless gift of salvation, and God requires nothing from us but to believe it. When we attempt to earn salvation, we are trying to put a price on something that is priceless.