What did it for me was this article in the New York Times, which clearly shows Christian symbols dating back 100 years before the purported existence of Christ: https://www.nytimes.com/1990/07/24/science/judean-cave-city-yields-trove-of-ancient-secrets.html

Judean Cave City Yields Trove of Ancient Secrets By Joel Brinkley July 24, 1990

The article included the photograph of a Crucifix:

Last year Professor Kloner's diggers came upon an especially enigmatic find in one newly discovered cave: the symbol of a cross, carved well before the time of Christ...

Yet there on the back wall, four feet high, two feet across, cut very deliberately six inches deep in the stone, is an unmistakable cross that looks very much like the Christian symbol. Nothing nearby explains its meaning, and Dr. Kloner declines to venture a guess.

''Clearly it has nothing to do with the Christians,'' he said, since it was cut at least 100 years before Christ was born. Crucifixion was practiced here in at least part of the period when the cave was in use, he noted.

He would offer only one vague suggestion: that the cross ''is probably some sort of cultic schematic of the Edomites.'' These were a Semitic people who had no clear religious orientation.