The “material principle” of the Reformation was justification by faith alone. As the Westminster Confession of Faith says, “Faith, thus receiving and resting on Christ and his righteousness, is the alone instrument of justification: yet is it not alone in the person justified, but is ever accompanied with all other saving graces, and is no dead faith, but worketh by love.”
The Genevan Confession likewise pointed out the necessity of those justified living by faith saying,
We confess that the entrance which we have to the great treasures and riches of the goodness of God that is vouchsafed us is by faith; inasmuch as, in certain confidence and assurance of heart, we believe in the promises of the gospel, and receive Jesus Christ as he is offered to us by the Father and described to us by the Word of God.
The Reformers never tired of saying that “justification is by grace alone, through faith alone, because of Christ alone.” When put into theological shorthand the doctrine was expressed as “justification by faith alone,” the article by which the church stands or falls, according to Martin Luther. The Reformers called justification by faith Christianity’s “material principle” because it involves the very matter or substance of what a person must understand and believe to be saved. Justification is a declaration of God based on the work of Christ. It flows from God’s grace and it comes to the individual not by anything he or she might do but by “faith alone” (sola fide). We may state the full doctrine as: Justification is the act of God by which he declares sinners to be righteous because of Christ alone, by grace alone, through faith alone.