Today the cross is the primary icon of Christianity, but I'm aware of a few others, such as the Ichthys. What symbols have represented the faith through the centuries?
An anchor was used by the early christians as a symbol of faith. "as an anchor of the soul, sure and firm" (Hebrews 6:19-20). There are several variations some of them can be seen in jesuswalk.com
Another very famous symbol is the chi and rho. Which was used by emperor Constantine as a heavenly divine symbol (coeleste signum dei) on the shields of his soldiers defeating Maxentius in the Battle of Milvian Bridge. For a depiction look at this brief article of Rochester University. The chi and rho are also believed to be reference to the first letters of the word Christ.
Another beatiful symbol is the lamb of god, which makes reference to the imagery of the book of revelation. The meaning of this symbol is explained by John the Baptist himself. "Behold the Lamb of God who takes away the sin of the world" in John 1:29. I think the most beautiful of this depictions can be seen in the wikipedia
This list does not pretend to be exhaustive.