Again, all sides agreed to the necessity of faith for salvation. But is faith alone sufficient? Do our works contribute anything? Our baptism? Our penance? Just what does God require?
The “material principle” of the Reformation was this principle of “Faith alone,” and it follows from solus Christus and sola gratia. If Christ does the work that saves so that we are saved by grace alone, then human works and merit are excluded: salvation is by faith alone. With every other means of approach to God stripped away, and recognizing our utter inability to contribute anything to our own salvation, we abandon ourselves to Christ, resting in him (alone!) to save us.
Here B.B. Warfield shows the relationship of faith and Christ and salvation:
If, with even greater explicitness still, faith is sometimes said to rest upon some element in the saving work of Christ, as, for example, upon His blood or His righteousness (Rom. 3:25, 2 Pet. 1:1), obviously such a singling out of the very thing in His work on which faith takes hold, in no way derogates from its repose upon Him, and Him only, as the sole and sufficient Saviour.
The saving power of faith resides thus not in itself, but in the Almighty Saviour on whom it rests. It is never on account of its formal nature as a psychic act that faith is conceived in Scripture to be saving,—as if this frame of mind or attitude of heart were itself a virtue with claims on God for reward, or at least especially pleasing to Him (either in its nature or as an act of obedience) and thus predisposing Him to favour, or as if it brought the soul into an attitude of receptivity or of sympathy with God, or opened a channel of communication from Him. It is not faith that saves, but faith in Jesus Christ: faith in any other saviour, or in this or that philosophy or human conceit (Col. 2:16, 18, 1 Tim. 4:1), or in any other gospel than that of Jesus Christ and Him as crucified (Gal. 1:8, 9), brings not salvation but a curse. It is not, strictly speaking, even faith in Christ that saves, but Christ that saves through faith. The saving power resides exclusively, not in the act of faith or the attitude of faith or the nature of faith, but in the object of faith; and in this the whole biblical representation centers, so that we could not more radically misconceive it than by transferring to faith even the smallest fraction of that saving energy which is attributed in the Scriptures solely to Christ Himself. (“Biblical Doctrine of Faith” in The Works of Benjamin B. Warfield, Vol. 2, p. 503–4)