John Gill

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.