Machen on 'Do This and Live'
"'The law is not of faith (does not partake of the nature of faith), but as Scripture says, 'He who does them shall live in (or by) them.' Paul means to say, "describing the nature of the law, it requires doing something. But faith is the opposite of doing. So when the Scripture says that a man is justified by faith, that involves saying that he is not justified by anything that he does. There are two conceivable ways of salvation. One way is to keep the law perfectly, to do the things which the law requires. No mere man since the fall has accomplished that. The other way is to receive something, to receive something that is freely given by God's grace. That way is followed when a man has faith. But you cannot possibly mingled the two. You might conceivably be saved by works or you might be saved by faith; but you cannot be saved by both. It is 'either or' here not "both and." - J. Gresham Machen Notes on Galatians p. 178