When we think about the Protestant Reformation we usually think about Martin Luther. But, there was one important reformer that is often forgotten. Andreas Karlstadt. Karlstadt was a personal friend and co-worker with Martin Luther but strenuously opposed him on the Sabbath issue. Luther himself admitted that Karlstadt was his superior in learning. Quoting Dr. White, Bishop of Ely: "the observance of the seventh day was being revived in Luther’s time by Karlstadt" (Treatise of the Sabbath, page 8). And from Sears Life of Luther, page 402: "Karlstadt held to the Divine authority of the Sabbath from the Old Testament." Indeed Luther says (in his book Against the Celestial Prophets): "Indeed, if Karlstadt were to write further about the Sabbath, Sunday would have to give way, and Sabbath—that is to say, Saturday—must be kept holy.