Harrison, Harry, and Tom Shippey. "Letter from the Pope"
Divergence: 878 CE
What if: The last Christian king in England broke with the church.
Summary: In 878, Alfred receives the letter from the pope that pushes him over the edge.
Published: In What Might Have Been? Volume 2: Alternate Heroes (eds. Gregory Benford and Martin H. Greenberg), q.v.; and The Mammoth Book of Alternate Histories (eds. Ian Watson and Ian Whates), q.v.