If We Are Made Heirs by the Law, Then Faith is Empty and the Promise is Meaningless
Thinking about Romans 4:14-15.
For if those who are of the law are heirs, faith is made void and the promise made of no effect, because the law brings about wrath; for where there is no law there is no transgression. (Romans 4:14-15)
Paul has just reminded the Christians in Rome that the promise to Abraham was not through the law but through the righteousness of faith.
Now he goes on to make the point that if we are to receive the promise through the law, then faith is “made void,” and the promise of God is meaningless.
If one can be saved by keeping a law system, then what need is there in faith? Faith would have no part in being an heir. - Wacaster, Studies in Romans, 165
Whether “the law” refers to the Law of Moses or to living flawlessly, the point is the same - if you could “earn” salvation by perfectly keeping the law, then you don’t need faith, and you don’t need a promise!
Paul emphatically rejected the Law as a means of gaining acceptance with God, now that Jesus the Messiah has come. For him, there was no alternative to gaining God’s inheritance except through faith (see Galatians 3:18). Abraham inherited what God had promised because he trusted His promises, and the same is true for all who desire to follow God. - Pollard, Truth for Today Commentary, 149
We’ve already noticed that the person who doesn’t need God’s grace is the person who has never sinned - who has lived a sinlessly perfect life.
Paul is pointing out to his audience that the promise to Abraham was through the righteousness of faith, which implies that we could not live a sinlessly perfect life - “all have sinned” and have fallen short of God’s glory (Romans 3:23).
Therefore, “faith” and the “promise” is necessary to spare us from the wrath we deserve by the law.