14 For if they which are of the law be heirs, faith is made void, and the promise made of none effect: (Romans 4:14 KJV)
William Burkitt’s Commentary
That is, If they which seem to be justified by the works of the law are heirs of this promised inheritance; then faith, that is, the way of justification by faith, prescribed by God, is to no purpose, for to what end should we by faith seek righteousness in another, if by our legal obedience we can find ourselves?
Here then he couched another argument, to confirm the apostle’s doctrine of justification by faith: Thus, that only justifies, unto which gracious promise of justification is made; but no such promise is made to any man for his weak and imperfect keeping of the law, but for his believing there is; therefore, by the law, there can be no justification, but by faith only.