Early Christianity was a minority group. Up until the fourth century, it was not considered a mainstream view and was commonly persecuted. They were not really accepted by any group, including the Romans and Jews. So, they often did things secretly and communicated in code. We see this in the number of symbols used by Christians to communicate with each other.
After the Edict of Milan in 313 AD, giving Christians the right to worship Christ, Christian symbols started to appear openly on many objects. Until the Edict, Christians had worshipped in secret and displayed their symbols in the underground Catacombs.
Back in the early stages of Christianity, within the first three centuries after Christ in 30 CE, the Romans were persecuting the Christians and killing them by the hundreds. To keep themselves safe, Christians developed secret symbols to allow other Christians to know about their faith without exposing themselves to the Romans for their own safety. An example of this would be to draw half of a symbol on the ground for the other person to see. If the second person completed the symbol, the first person knew that they were also a Christian.
What was early Christianity like? We the archaeology of the Roman catacombs in the second and third centuries AD, give us clues, where the graves of hundreds of thousands of Christians give clues, especially with a number of symbols that reflect the earliest period of the Christian era.
Most of these represent Christian visual art prior to 313 A.D., when Emperor Constantine legalized Christianity. During this time, the cross is seldom seen, except disguised in some way as an anchor, a trident, or the mast of a ship. Instead, Christians identified their tombs in the catacombs by other symbols alongside of funerary inscriptions.