Cardinal Robert Bellarmine (1542–1621), Pope Clement VIII’s personal theologian and one of the most able figures in the Counter-Reformation movement within sixteenth-century Roman Catholicism, once wrote: “The greatest of all Protestant heresies is _______ .” What do you think he said? This article is about assurance of faith (assurance of salvation). The author also discusses Hebrews 10:18-22, justification by faith, and good works.