For if they who are of the law, etc. He takes his argument from what is impossible or absurd, that the favor which Abraham obtained from God, was not promised to him through any legal agreement, or through any regard to works; for if this condition had been interposed -- that God would favor those only with adoption who deserved, or who performed the law, no one could have dared to feel confident that it belonged to him: for 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 fulfillment of the promises would be rendered void; for they avail nothing but when received by faith. If our adversaries had ears to hear this one reason, the contest between us might easily be settled. The Apostle assumes it as a thing indubitable, that the promises would by no means be effectual except they were received with full assurance of mind. But what would be the case if the salvation of men was based on the keeping of the law? consciences would have no certainty, but would be harassed with perpetual inquietude, and at length sink in despair; and the promise itself, the fulfillment of which depended on what is impossible, would also vanish away without producing any fruit. Away then with those who teach the common people to seek salvation for themselves by works, seeing that Paul declares expressly, that the promise is abolished if we depend on works. But it is especially necessary that this should be known, -- that when there is a reliance on works, faith is reduced to nothing. And hence we also learn what faith is, and what sort of righteousness ought that of works to be, in which men may safely trust. The Apostle teaches us, that faith perishes, except the soul rests on the goodness of God. Faith then is not a naked knowledge either of God or of his truth; nor is it a simple persuasion that God is, that his word is the truth; but a sure knowledge of God's mercy, which is received from the gospel, and brings peace of conscience with regard to God, and rest to the mind. The sum of the matter then is this, -- that if salvation depends on the keeping of the law, the soul can entertain no confidence respecting it, yea, that all the promises offered to us by God will become void: we must thus become wretched and lost, if we are sent back to works to find out the cause or the certainty of salvation.

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.

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.

(12) For if they which are of the (k) law [be] heirs, faith is made void, and the promise made of none effect:
(12) A double confirmation of that reason: the one is that the promise cannot be apprehended by the law, and that if it could it would be made of no effect: the other, that the condition of faith would be joined in vain to the promise if it could be apprehended by works.
(k) If they are heirs who have fulfilled the law.

For if they which are of the law be heirs,.... That is, if the Jews who are under the law, and are seeking for righteousness and life by the works of it, should, on the account of their obedience to it, be heirs of the grace of life and of glory,
faith is made void; for if the right to the inheritance is by the works of the law, there is no room for faith; that can be of no use or service;
and the promise made of none effect: if salvation is by works, it is to no purpose for God to promise, or men to believe; for the thing promised depends not upon God's promise, but upon man's obedience to the law; and if that is not perfectly observed, as it cannot possibly be, then the promise of God stands for nothing, and is in course made void. The apostle here argues from the absurdities which follow upon the doctrine of justification by works, as he does from the different effects of the law, in the following verse.

For if they which are of the law be heirs--If the blessing is to be earned by obedience to the law.
faith is made void--the whole divine method is subverted.

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.