“For if it is the adherents of the law who are to be the heirs, faith is null and the promise is void.” -Romans 4:14
Raising a hypothetical situation in order to show its ineffectiveness, Paul will conclude that faith is dead that rests in keeping the law instead of the goodness of God. And, if there is no faith—if it is in effect null and void—then there would be no way to claim the promise of heirship because faith is the instrument by which the promise is claimed.
John Calvin’ s note on this passage, though incomplete and, therefore, potentially misleading in my opinion, is nevertheless instructive. He writes,
who is there so conscious of so much perfection that he can feel assured that the inheritance is due to him through the righteousness of the law? Void then would faith be made; for an impossible condition would not only hold the minds of men in suspense and anxiety, but fill them also with fear and trembling: and thus the fulfilment of the promises would be rendered void; for they avail nothing but when received by faith.
Though what he says is true as far as it goes, and the mental picture he provides is helpful, it almost sounds as if he is saying faith needs to be mustered by the recipient of the promise in order for it to be effective; therefore, trusting in God’s goodness is a better choice than trusting in our ability to keep the law. I don’t think Calvin actually believed that since in other places he affirms faith is a gift of God, an instrument by which the promise can be received rather than a scaling merit system.
What Paul is setting out to prove is that faith and adhering to the Law are mutually exclusive. As verse 15 will demonstrate, the law brings wrath, while Abraham’s faith was in a gift freely given. The former directs faith toward self-righteousness while the latter directs faith toward God’s righteousness. Only faith in God’s righteousness can apprehend the promise because faith in our own righteousness (our ability to keep the law) is null making the promise void.