Sola fide

Sola fide, meaning faith alone, is a Protestant Christian doctrine that teaches sinners are forgiven and declared “not guilty” — by God's grace through faith, not by good works or religious deeds.

Traditional Protestants believe that this doctrine of salvation is the cornerstone of Christianity, the very teaching "upon which the church stands or falls". This belief clearly distinguishes Lutheran and Reformed Protestant churches from Catholic, Eastern Orthodox, Oriental Orthodox, Assyrian, Methodist and Anabaptist traditions.

In Lutheran and Reformed theologies, good works show true faith but do not contribute to salvation. Confessional Lutherans, for example, see justification as God's free forgiveness. In contrast, Methodist doctrine teaches that while justification comes through faith, salvation also requires a life of holiness aimed at entire sanctification, maintained by continued faith and obedience. Anabaptists reject sola fide, stressing a transformative journey where "justification [began] a dynamic process" helping believers grow to reflect Christ.

Roman Catholicism teaches that salvation comes through both faith and works, holding to the concept of fides formata — faith formed by charity. Unlike the Protestant doctrine of sola fide (faith alone), Catholic theology emphasizes that faith must be accompanied by personal "merit" and the "observance of the commandments." Eastern Orthodoxy shares a similar view, teaching that salvation requires both faith and the sinner’s active cooperation. Thus, various Christian churches believe that faith and good works are each necessary but not sufficient for salvation in the afterlife: they hold that both are required. They have often disparaged the idea that good works alone are enough, which they have often called legalism.

Although modern Catholic scholars are against Luther's use of the word "only", Catholic sources before Refomation had done the same. In 1916, Lutheran scholar Theodore Engelder published an article titled "The Three Principles of the Reformation: Sola Scriptura, Sola Gratia, Sola Fides" ("only scripture, only grace, only faith").

Martin Luther elevated sola fide to the principal cause of the Protestant Reformation, the rallying cry of the Lutheran cause, and the chief distinction of the Lutheran and Reformed branches of Christianity from Roman Catholicism.

Luther added the word allein ("alone" in German) to Romans 3:28 controversially so that it read: "So now we hold, that man is justified without the help of the works of the law, alone through faith". The word "alone" does not appear in the Greek manuscripts and Luther acknowledged this fact, but he defended his translation by maintaining that the adverb "alone" was required by idiomatic German:

I knew very well that the word solum ["alone" in Latin] is not in the Greek or Latin text (…) It is a fact that these four letters S O L A are not there (…) At the same time (…) it belongs there if the translation is to be clear and vigorous. I wanted to speak German, not Latin or Greek, since it was German I had undertaken to speak in the translation. But it is the nature of our German language that in speaking of two things, one of which is affirmed and the other denied, we use the word solum (allein) along with the word nicht [not] or kein [no]. For example, we say, 'The farmer brings allein [only] grain and kein [no] money.