MANBIJ, Syria – For more than two years, ISIS forces who occupied this northern Syrian city paid little attention to the tip of an old gate on an empty mound of land where they dumped trash. They were clueless the gate ran several feet into the ground down to something they might well have destroyed had they known: The ruins of an ancient Christian refuge, or early church, possibly dating back to the first centuries of Christendom’s existence, under the Roman Empire. “I was so excited, I can’t describe it. I was holding everything in my hands,” Abdulwahab Sheko, head...