Probably the most commonly used symbol by early Christians was the fish. It first appeared in Christian art and literature in the second century AD, then spread widely during the third and fourth centuries. It is still frequently used today.
In Greek the word for fish is Ichthus, consisting of five letters. These are used as an acrostic with this meaning:
I = Iesous (Jesus)
Ch = Christos (Christ)
Th = Theou (God's)
Y = Huios (Son)
S = Soter (Saviour)
During times of persecution, the symbol of a fish was used by Christians to identify fellow believers. One person would draw a curved line in the sand - a shape which would have no
meaning to the authorities. Then the other person would draw another curved line, thus completing the shape of a fish.