What Day Changed the Course of Christian History?

The Fall of Constantinople, May 29, 1453

 An interesting question asked of four leading Christian historians by The Gospel Coalition--the exact question was, "After AD 70, what day most changed the course of Christian history?" Robert Wilken of the University of Virginia thinks that the Muslim invasions of the Middle East in the seventh century was the most impactful event. George Marsden of Notre Dame says that it was the conversion of Constantine in the early 4th century. Philip Jenkins of Baylor University opines that it was the fall of Constantinople in 1453. And Thomas Kidd, also of Baylor, points to the day when George Whitefield preached at Jonathan Edwards' church in 1740, beginning the evangelical movement in America.

See the full responses here, and feel free to comment on how you would answer the question.