Fish
Jesus was linked to fish through his miracle of the loaves and fishes, and because he called his disciples "fishers of men." A century after his death, converts linked fish to sacred Christianity by making an acrostic from the Greek for it, ichthys: Iesous Christos Theou Yios Soter, or: Jesus Christ, Son of God, Savior. Since it was already a sacred icon for Greeks, Romans, and other pagans, a fish marking didn’t raise suspicion, so early Christians in danger of persecution used its stylized image to signal meeting places, tombs, and allies. If a Christian met a stranger on the road, he sometimes drew in its dirt one arc of the iconic outline, and if the stranger stooped and drew the completing arc, both knew they were safe among true believers. Now drivers show other cars a fish bumper sticker.