In his commentayr on Revelation, James Durham says: "1. That the person suffer as a well-doer, so it must be for the truth of Christ, or righteousness sake (Matt. 5:10, 11); for, non est mors, sed causa mortis quæ facit Martyrem [it is not the dying, but the cause of death that makes the martyr]." Citing Latin like this usually implies a standard saying. I've struck out finding anything except two unsourced uses of the phrase, one being a commentary on Revelation so it likely had Durham as the source. If anyone dabbling in the Latin fathers has any other ideas, this is my current resolution, that Durham was citing from memory and miss cited a saying of Augustine. Thoughts? Note below.
Fn. This may simply be an old maxim as J. A. Seiss styles it (I {1865; 8th ed., 1901}, 350). Seiss does not credit it otherwise, but he may have drawn it from here. The phrasing may be Durham’s, and he may have had in mind Augustine’s Non facit martyrem pœna, sed causa (It is not the punishment but the cause that makes the martyr). See Augustine, Opera Omnia, Sermones, Classis III, Sermones de Sanctis, Sermo CCCXXVII, Migne, PL 38, col. 1451.
Fn. This may simply be an old maxim as J. A. Seiss styles it (I {1865; 8th ed., 1901}, 350). Seiss does not credit it otherwise, but he may have drawn it from here. The phrasing may be Durham’s, and he may have had in mind Augustine’s Non facit martyrem pœna, sed causa (It is not the punishment but the cause that makes the martyr). See Augustine, Opera Omnia, Sermones, Classis III, Sermones de Sanctis, Sermo CCCXXVII, Migne, PL 38, col. 1451.