We just learned about Staurogram.
Another Christian symbol is the Chi-Rho.
In the Greek language, the name for Christ was spelled ΧΡΙΣΤΟΣ, and sounded like "Christos".
Χ - Chi
Ρ - Rho
Ι - Iota
Σ - Sigma
Τ - Tau
Ο - Omicron
Σ - Sigma
The first two letters Chi and Rho, ΧΡ, were put together as a symbol to mean Christ.
Over 1,500 years ago one of the Emperors of Rome named Constantine became a Christian, and he had his armies put the Chi Rho symbol on their shields and armor to show that he was fighting for Christ.

(from: wikipedia - chi rho)
Kid Facts - Blast from the past: Choosing of Stephen