Imagine the dangerous life of an early Christian. You've embraced your newfound faith in Christ but fear the risk of persecution or death at the hands of the pagans living around you. Then a trusted friend tells you about some of Jesus' followers who secretly meet. He whispers into your ear, "Look for a fish carved in a paving stone" by a certain home on the Via Tiburtina. You smile in gratitude. Still today, modern society recognizes those...