What is the earliest representation of the cross as a Christian symbol?

Here’s a decent discussion of the history of the cross in the context you are asking about.

There supposedly was a “fish” symbol found on several stones in Herculaneum, which was destroyed in 79 AD.

It’s an interesting article. Crucifixion as well as being a very cruel death was a shameful and ignominious one and the very earliest Christians didn’t like to use the cross as a symbol of their faith. @Tropical_Willie Yes, Christians had many symbols to represent their faith but I am particularly interested in the cross which has become the most important one.

The early Christians in Rome could not display the Cross under penalty of death. Therefore they used the fish and other symbols.

This mocking drawing showing the crucifixion of a person with a donkey’s head is possibly the oldest representation. Wikipedia mentions it in this article.