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.
James Montgomery Boice, Whatever Happened to the Gospel of Grace? (Wheaton, Ill.: Crossway, 2001), pp. 65-149.