Martyrdom has always been a proof of the intensity, never of the correctness, of a belief.

[Das Martyrium war immer nur ein Beweis für die Intensität, niemals für die Richtigkeit eines Glaubens.]

Arthur Schnitzler (1862-1931) Austrian physician, playwright, novelist

Book of Thoughts and Sayings [Buch der Sprüche und Bedenken], “Hunches and Questions” No. 2 (1927)
    (Source)

(Source (German)). See Ingersoll (1881). This translation is common, but unattributed. Other translation:

Martyrdom was only ever a proof of the intensity, never of the correctness of a belief.
[E.g.]