Romans 4:14

For, if they which are of the law be heirs - If the Jews only be heirs of the promise made to Abraham, and that on the ground of prior obedience to the law, then faith is made void - is entirely useless; and the promise, which was made to faith, is made of none effect.

For if they which are of the law - Who seek for justification and acceptance by the Law.

Faith is made void - Faith would have no place in the scheme; and consequently the strong commendations bestowed on the faith of Abraham, would be bestowed without any just cause. If people are justified by the Law, they cannot be by faith, and faith would be useless in this work.

And the promise ... - A promise looks to the future. Its design and tendency is to excite trust and confidence in him who makes it. All the promises of God have this design and tendency; and consequently, as God has given many promises, the object is to call forth the lively and constant faith of people, all going to show that in the divine estimation, faith is of inestimable value. But if people are justified by the Law; if they are rendered "acceptable" by conformity to the institutions of Moses; then they cannot depend for acceptance on any promise made to Abraham, or his seed. They cut themselves off from that promise, and stand independent of it. That promise, like all other promises, was made to excite faith. If, therefore, the Jews depended on the Law for justification, they were cut off from all the promises made to Abraham; and if they could be justified by the Law, the promise was useless. This is as true now as it was then. If people seek to be justified by their morality or their forms of religion, they cannot depend on any promise of God; for he has made no promise to any such attempt. They stand independently of any promise, covenant, or compact, and are depending on a scheme of their own; a scheme which would render his plan vain and useless; which would render his promises, and the atonement of Christ, and the work of the Spirit of no value. It is clear, therefore, that such an attempt at salvation cannot be successful.

4:14 If they only who are of the law - Who have kept the whole law. Are heirs, faith is made void - No blessing being to be obtained by it; and so the promise is of no effect.