Some quotes:

“Justification by faith alone is the article of the standing or falling Church.”

“Whoever departs from the article of justification does not know God and is an idolater . . . For when this article has been taken away, nothing remains but error, hypocrisy, godlessness, and idolatry, although it may seem to be the height of truth, worship of God, holiness, etc. . . If the article of justification is lost, all Christian doctrine is lost at the same time.”

“When the article of justification has fallen, everything has fallen. Therefore it is necessary constantly to inculcate and impress it, as Moses says of his Law (Deut. 6:7); for it cannot be inculcated and urged enough or too much. Indeed, even though we learn it well and hold to it, yet there is no one who apprehends it perfectly or believes it with a full affection and heart. So very trickish is our flesh, fighting as it does against the obedience of the spirit.”

“This doctrine [justification by faith alone] is the head and the cornerstone. It alone begets, nourishes, builds, preserves, and defends the church of God; and without it the church of God cannot exist for one hour. For no one who does not hold this article or, to use Paul’s expression, this ‘sound doctrine’ (Titus 2:1) is able to teach aright in the church or successfully to resist any adversary . . . this is the heel of the Seed that opposes the old serpent and crushes its head. That is why Satan, in turn, cannot but persecute it.” – Martin Luther

Back in the 16th century, the Roman Catholic Church believed (then as it does now) that justification is by grace, through faith and because of Christ. What Rome does not believe is that justification is by faith alone, or by grace alone, or by Christ alone. For Rome, justification is by grace plus merit, through faith plus works; by Christ plus the sinner’s contribution of inherent righteousness. In contrast, Martin Luther and the Reformers called the Church back to the one true Biblical Gospel: Salvation is by God’s grace alone, received through faith alone, because of Christ alone, based on the Scriptures alone, to the Glory of God alone.