Fueled by strife, civil war and the rise of extremist groups, the exodus deepens the dominance of Islam.

Like the Jews before them, Christians are fleeing the Middle East, emptying what was once one of the world’s most-diverse regions of its ancient religions. They’re being driven away not only by Islamic State, but by governments the U.S. counts as allies in the fight against extremism. When suicide bomb attacks ripped through two separate Palm Sunday services in Egypt last month, parishioners responded with rage at Islamic State, which claimed the blasts, and at Egyptian state security. Government forces assigned to the Mar Girgis church in Tanta, north of Cairo, neglected to fix a faulty metal detector at the entrance after church guards found a bomb on the grounds just a week before. The double bombing killed at least 45 people, and came despite promises from the Egyptian government to protect its Christian minority.

The exodus leaves the Middle East overwhelmingly dominated by Islam, whose rival sects often clash, raising the prospect that radicalism in the region will deepen. Conflicts between Sunni and Shiite Muslims have erupted across the Middle East, squeezing out Christians in places such as Iraq and Syria and forcing them to carve out new lives abroad, in Europe, the U.S. and elsewhere. “The disappearance of such minorities sets the stage for more radical groups to dominate in society,” said Mr. Johnson of the loss of Christians and Jews in the Middle East. “Religious minorities, at the very least, have a moderating effect.”

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