How do you know?
Luther and Calvin substituted their own understanding for that of the Church founded by Jesus.
Luther even went so far as to eliminate books of the bible and to change words of the Holy Scriptures.
Interesting quotes from Martin Luther:
Although Martin Luther stated that he looked upon the Bible “as if God Himself spoke therein” he also stated,
My word is the word of Christ; my mouth is the mouth of Christ" (O’Hare PF. The Facts About Luther, 1916–1987 reprint ed., pp. 203-204).
The Bible, in Romans 3:28, states,
Therefore we conclude that a man is justified by faith apart from the deeds of the law.
Martin Luther, in his German translation of the Bible, specifically added the word “allein” (English ‘alone’) to Romans 3:28-a word that is not in the original Greek. Notice what Protestant scholars have admitted:
…Martin Luther would once again emphasize…that we are “justified by faith alone”, apart from the works of the Law" (Rom. 3:28), adding the German word allein (“alone”) in his translation of the Greek text. There is certainly a trace of Marcion in Luther’s move (Brown HOJ. Heresies: Heresy and Orthodoxy in the History of the Church. Hendrickson Publishers, Peabody (MA), 1988, pp. 64-65).
Furthermore, Martin Luther himself reportedly said,
You tell me what a great fuss the Papists are making because the word alone in not in the text of Paul…say right out to him: ‘Dr. Martin Luther will have it so,’…I will have it so, and I order it to be so, and my will is reason enough. I know very well that the word ‘alone’ is not in the Latin or the Greek text (Stoddard J. Rebuilding a Lost Faith. 1922, pp. 101-102; see also Luther M. Amic. Discussion, 1, 127).
cogwriter.com/luther.htm