I understand WHY the cross became a symbol for Christianity, but HOW did it become a symbol? How did it start and how did it evolve into the symbol (or symbols since there are many variants) we all know today?
The way it was explained to me once is that it was a literal interpretation of Christ's commission to take up the cross and follow him.
The Encyclopedia Britannica attributes Emperor Constantine as the one that initiated the use of the cross as a symbol of Christianity:
I'm not sure if this is how it came about but it could have contributed to it. This was told to me by a friend so I don't have sources for it right now. I'll look for sources on this later.
In the early church days Christians were persecuted by both Jews and Romans. In order to keep their worship services safe and secure they needed to develop a signal to inform everyone that they were 'safe' and not there to harm everyone or be a spy. So they used the hand gesture of making the sign of the cross on their bodies, the same sign that many Christian faiths use today while praying.
If you were outside of the church and looked in, you couldn't see what was being done or what sign was made. However, if you were inside the church, you could watch people as they came in to determine if they were a Christian or not. Anyone coming in who did not know the sign would be noticed quite easily.