Several years ago, I recovered a Saint Christopher medal from the grounds of a WW2 training area in middle Tennessee. Immediately intrigued by the medal, its meaning, and why a solider would have worn it, I took a closer look at the relationship between Saint Christopher and the military.
The most recognized Christian symbol is most certainly the simple sign of the Cross. Such symbols were prevalent in the early era of the Church, especially in the times of Christian persecution. Used to identify oneself as belonging to Christ, symbols were akin to a Christian password in times of persecution.