Source: The Washington Post By E.J. Dionne Jr. Christmas remains wondrous, but it arrives at a difficult moment for Christianity in the United States. We still see Christmas trees strapped to the tops of cars, neighbors lighting up their homes, and children getting as excited as ever. And the churches will be unusually full. This last point is revealing: A relative decline of religious observance has brought forth the “Chreasters,” Christians who attend services only on Christmas and Easter. Regular worshipers can be disdainful of the Chreasters. They make it hard for the loyalists to find seats in the pews and…