16 Therefore it is of faith, that it might be by grace; to the end the promise might be sure to all the seed; not to that only which is of the law, but to that also which is of the faith of Abraham; who is the father of us all, (Romans 4:16 KJV)
William Burkitt’s Commentary
The apostle here assigns a double cause, why the wisdom of God has appointed justification and salvation to be obtained in the way of faith:
namely, 1. That it might be of free and undeserved grace and favour; for to be justified by faith and by grace are all one with the apostle.
And, 2. That the promise might be sure to all the seed: That is, that God’s promise might stand firm and sure to all the believing seed of Abraham, not only to all the children of the flesh, to whom the law was given, but to all the children of his faith, even Gentiles as well as Jews; he being the father of all that believe, whether Jews or Gentiles.
Learn hence, That if our justification and salvation did depend on our performing perfect obedience to the law, it would never be sure, but always uncertain, because of our impotency and weakness to keep and observe it.
The apostle, Ro 8:3, tells us, That the law is weak through the flesh; though the truth is, the law is not weak to us, but we are weak to it; the law has the same authority for commanding that ever it had, but we have not the same ability for obeying. ‘Tis our wickedness that is the sole cause of the law’s weakness: Had every man the same integrity, the law would have the same ability that ever it had, both to justify and save us.