The church of the Middle Ages spoke about Christ. A church that failed to do that could hardly claim to be Christian. But the medieval church had added many human achievements to Christ’s work, so that it was no longer possible to say that salvation was entirely by Christ and his atonement. This was the most basic of all heresies, as the Reformers rightly perceived. It was the work of God plus our own righteousness. The Reformation motto solus Christus was formed to repudiate this error. It affirmed that salvation has been accomplished once for all by the mediatorial work of the historical Jesus Christ alone. His sinless life and substitutionary atonement alone are sufficient for our justification, and any “gospel” that fails to acknowledge that or denies it is a false gospel that will save no one.
James Montgomery Boice, Whatever Happened to the Gospel of Grace? (Wheaton, Ill.: Crossway, 2001), pp. 65-149.