"Christ, according to Paul, will do everything or nothing; if righteousness is in slightest measure obtained by our obedience to the law, then Christ died in vain; if we trust in slightest measure in our own good works, then we have turned away from grace and Christ profits us nothing.
To the world, that may seem to be a hard saying; it is not a hard saying to the man who has ever been at the foot of the cross; it is not a hard saying to the man who has first known the bondage of the law, the weary effort at establishment of his own righteousness in the presence of God, and then has come to understand, as in a wondrous flash of light, that Christ has done all, and that the weary bondage was vain.
What a great theologian is the Christian heart—the Christian heart that has been touched by redeeming grace! The man who has felt the burden of sin roll away at the sight of the cross, who has said of the Lord Jesus, 'He loved me and gave Himself for me,' who has sung with Toplady: 'Nothing in my hand I bring, Simply to Thy cross I cling'—that man knows in his heart of hearts that the apostle is right, that to trust Christ only for part is not to trust Him at all, that our own righteousness is insufficient even to bridge the smallest gap which might be left open between us and God, that there is no hope unless we can safely say to the Lord Jesus, without shadow of reservation, without shadow of self-trust: 'Thou must save, and Thou alone.'"
J. Gresham Machen, "What Is Faith?"