To demonstrate this, Catholic author Dave Armstrong ( a former Protestant campus missionary) focuses on those issues about which Catholics and Protestants disagree the most: the role of the Bible as a rule of faith, whether we are justified by faith alone, whether doctrine develops, what the Eucharist really is, veneration of Mary and prayer to the saints, the existence of purgatory, the role of penance in salvation, and the nature of infallibility of the papacy.