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Sunday, October 31, is Reformation Day. It marks 493 years since Martin Luther nailed his 95 theses to the famous church door in Wittenberg, Germany. The Augustinian monk set in motion a sequence of events that reverberated through Western Christendom and continues to mark and separate us today.

Since returning to the Catholic Church in late April 2007, I find Reformation Day has taken on a different meaning than when I stood on the other side of the Tiber. Nevertheless, even as a Protestant, my enthusiasm for October 31 never rose higher than modest appreciation for what I thought were Luther’s, and later Calvin’s, significant contributions...