Apart from the cross, the most ubiquitous symbol of Christianity is the ichthys, known to us as the Jesus Fish, and today it appears predominantly in its natural habitat -- car bumpers. The ichthys actually dates right back to ancient times, when Christianity was still an obscure sect, and considering that fish and fishing were frequently used as symbols in the Bible, you could argue that it's a more appropriate symbol for the teachings of Christ than the device used to torture and kill him.