John Calvin, Institutes of the Christian Religion (1536)—This work originally began as a short French text outlining Calvin's teachings on Reformed Christianity. It underwent many changes and revisions, so that by Calvin's death it was an imposing volume that was widely circulated among Reformed Protestants. Today, it still ranks as one of Protestantism's most important texts and has made Calvin "the Protestant Aquinas."

Catherine of Genoa, Treatise on Purgatory (1551)—This work recounted Catherine's vivid visions of purgatory, and was widely circulated in the late fifteenth and early sixteenth centuries. It helped to establish in many people's minds an image of purgatory, that realm of the afterlife where Christians suffered for their sins.

Catherine of Siena, Dialogue of Divine Providence (c. 1378)—This was one of the most important mystical texts of the later Middle Ages.

Desiderius Erasmus, Translation of the New Testament (1516)—Erasmus'...