Christians were very severely persecuted, often for doing nothing even close to wrong. To admit your Christianity meant practically asking to die. But amazingly, the Message of Christ was never stopped, and it even flourished during this dark time. The people of this day, because they were not allowed to speak even the name of Christ, needed a way to identify themselves. They needed to show others, even secretively, about their faith in the true God, and not in the idols of the leaders. So they would take a stick, and make the simple drawing of an open fish.
That was the symbol which came to be called the icthus fish. It was a very secret thing, and one had to use extreme caution with making or displaying the symbol in order not to be caught and persecuted.
