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 first chapter of John Stott's The Cross of Christ provides a history of the development of the use of the symbol of the cross in Christianity. I'll briefly summarize it here.
The earliest images used by Christians did not include the cross. Persecution required that they be circumspect, which meant that an image clearly associated with both Christ and execution could not be preferred. Thus, the earliest images included peacocks, doves, and particularly fish. The greek word for fish, ichthys, served as an ancronym for Iesus Christos Theos Huios Soter ("Jesus Christ, Son of God, Saviour"), which drew less attention from the uninitiated.
Second-century images included biblical scenes like Noah's ark, Abraham killing the ram, and the raising of Lazarus. These had a closer relationship to the redemption of Christ, but were still not obvious enough for the public at large to understand the reference.
Choosing the cross
Christians had a variety of emblems to choose from, such as the manger where Jesus was born, or the boat from which he taught, or the stone that rolled away from his grave. But, says Stott, Christians wanted to:
The cross image at this time was not a crucifix, that is, a cross with Jesus' body on it; such images apparently arrive in the sixth century. (for more on this, see When did the split in crufix/empty-cross symbolism occur?)
Early use of the cross
Stott argues that the cross was in use by the 2nd century. He interprets Tertullian's AD 200 writing, "we trace upon the forehead the sign," as being the sign of the cross (De Corona, chapter 3). He also presents the writing of Hippolytus (The Apostolic Tradition, AD 215):
Hippolytus, says Stott, was a conservative who would not have been describing rites and customs unless they were "already long-established," perhaps a generation or more.
Additional evidence from the early church comes from Cyprian, in the middle of the third century, who also appears to make reference to the symbol of the cross in his work De Lapsis.
Constantine famously propelled the use of the cross as a symbol when he reportedly saw a vision at the Battle of the Milvian Bridge. The vision showed a cross and the words, "Conquer by this sign." The subsequent victory inspired Constantine's conversion, and he began using the cross on the standards of his army. The cross thus became widely known across the Roman Empire as the symbol of Christianity.
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.