John Calvin Commentary Luke 10:28

"And he said unto him, Thou hast answered right: this do, and thou shalt live." — Luke 10:28 (ASV)

Do this, and you shall live. I have explained a little before how this promise agrees with freely bestowed justification by faith: for the reason God justifies us freely is not that the Law does not point out perfect righteousness, but because we fail in keeping it; and the reason it is declared impossible for us to obtain life by it is that it is weak through our flesh (Romans 8:3).

So then, these two statements are perfectly consistent with each other: that the Law teaches how men may obtain righteousness by works, and yet that no man is justified by works, because the fault lies not in the doctrine of the Law, but in men. It was Christ’s intention, meanwhile, to vindicate Himself from the slander which, He knew, was brought against Him—that He set aside the Law, insofar as it is a perpetual rule of righteousness.