As far as we know, Christians in the first three centuries didn’t use the cross as a symbol. The popular symbols were of a shepherd carrying a lamb on his shoulders, an anchor, a vine and the fish sign.
No one knows exactly what Jesus was crucified on – was it T shaped, or the traditional cross shape or just an upright pole.
The Greek word stauros literally means a stake and that is the word that is traditionally translated as cross and from it crucify. No word actually means cross.
The Emperor Constantine’s mother Helena is reputed to have found the “true cross” in Jerusalem three hundred years after the time of Jesus. That was a long time after the event. There had been at least two revolts in Judea in which hundreds of thousands were slaughtered and the city destroyed. Josephus tells us that under the siege of Jerusalem in 70AD the Romans often caught five hundred people a day, who in their desperation, were out foraging for something to eat and they crucified them as a warning to others but he says they were running out of wood to crucify them on. Stakes or crosses – we don’t know but crosses require a lot more wood than stakes.
Many traditions come from that time.
Eusebius tells of the vision that Constantine had when he saw a sign in heaven and Christ told him that “in this sign he would conquer”. Constantine adopted this sign and went on to win his battles. Artists like Raphael have shown this sign as a cross but the historian Eusebius shows it as the first two letters for the Greek word for Christ – XP chi rho. Tradition, time and again triumphs over truth, fables and fantasies over facts.
The word for fish in Greek is Ichthys ΙΧΘΥΣ which stands for :–
of God Θ
Easily drawn with two curved lines or one line drawn by one stranger and completed by another as a “Christian password”. John wrote in his Gospel that he had written these things to show that Jesus is the Christ, the son of God and that through believing they might have life. So this was the statement of the Christian beliefs condensed into a rebus -the word for fish.