Salvation that is of God alone, by grace alone, through faith alone, is ours only in Christ alone. In On the Freedom of a Christian, Martin Luther underscored the “incomparable benefit” of faith in Christ, namely, union with Him:
“Faith unites the soul with Christ as a bride is united with her bridegroom… it follows that everything they have they hold in common, the good as well as the evil. Accordingly the believing soul can boast and glory in whatever Christ has as though it were its own, and whatever the soul has Christ claims as his own. Let us compare these and we shall see inestimable benefits. Christ is full of grace, life, and salvation. The soul is full of sins, death, and damnation. Now let faith come between them and sins, death, and damnation will be Christ’s, while grace, life, and salvation will be the soul’s; for if Christ is a bridegroom, he must take upon himself the things that are his bride’s and bestow upon her the things that are his. If he gives her his body and very self, how shall he not give her all that is his? And if he takes the body of the bride, how shall he not take all that is hers?… Who can understand the riches of the glory of this grace?
This excerpt is adapted from Sinclair Ferguson's contribution to the Legacy of Luther edited by R.C. Sproul and Stephen Nichols.