What does it mean to believe in Jesus Christ? What is faith? The Westminster Shorter Catechism defines faith as follows:
Q. 86. What is faith in Jesus Christ?
A. Faith in Jesus Christ is a saving grace, whereby we receive and rest upon him alone for salvation, as he is offered to us in the gospel.
So saving faith is faith that ‘receives and rests upon’ Christ alone for salvation. And true saving faith receives and rests upon Christ “as he is offered to us in the gospel.” It must be said that much of what often passes as preaching of the gospel of Christ does not fit that description. For how is Christ offered to us in the gospel? Is Christ offered as the Savior from the penalty of sin only, or as the Savior from sin – from its penalty, power, and (in the life to come), even its very presence?
The Scriptures plainly tell us that Jesus came to save His people from their sins (Matthew 1:21), and that His gospel is sent forth to offer both forgiveness and sanctification (Acts 26:18). Clearly, then, Christ is offered to us in the gospel both for our justification as well as our sanctification, and He must be received as such.
Calvin (in commenting on Romans 8:13), puts it this way:
“It is, indeed, true, that we are justified in Christ by the mercy of God alone, but it is equally true and certain, that all who are justified are called by the Lord to live worthy of their vocation. Let believers, therefore, learn to embrace Him, not only for justification, but also for sanctification, as He has been given to us for both these purposes, that they may not rend him asunder by their own mutilated faith.” (Calvin’s New Testament Commentaries, Vol. 10, p.167)
Those who would believe in Christ for justification alone (i.e. for forgiveness and acceptance before God as righteous as in His sight), but not also for sanctification, have a (to use Calvin’s phrase) “mutilated faith” that effectively seeks to ‘rend Christ asunder’ (or split Him in two). But a divided faith in a divided Christ saves no one. So let us learn, as Calvin says, to embrace Christ for sanctification as well as justification, for “he has been given to us for both these purposes.”