In the dusty highlands of northern Ethiopia, a team of archaeologists recently uncovered the oldest known Christian church in sub-Saharan Africa, a find that sheds new light on one of the Old World’s most enigmatic kingdoms—and its surprisingly early conversion to Christianity. An international assemblage of scientists discovered the church 30 miles northeast of Aksum, the capital of the Aksumite kingdom, a trading empire that emerged in the first century A.D. and would go on to dominate much of eastern Africa and western Arabia. Through radiocarbon dating artifacts uncovered at the church, the researchers concluded that the structure was built...