The fish symbol, Ichthys (Greek acronym), was used by christians near the beginning to identify themselves to other christians when their then-heathen religion was frowned upon by the powers of the day. It probably stems from Jesus's disciples being fishermen as well as other reasons. Batman must be religious - "Holy mackerel!" Incidentally, the fish symbol dated back to ancient religions as well.
The symbol that is popular now, the cross, stemmed from the Egyptian ankh. This was not a christian symbol for around 2-3 centuries I think. There are strong possibilites that a Jesus was not hung on a cross. The ancient Greek word for cross back when the bible was written actually meant stake. Either way, they believe he was killed. The cross obviously makes a far better symbol than a stake.
Damn(!), I know too much about religions, and believe in none!