Actually the history of the use of the cross as a symbol is better known than a lot of stuff. And, we are discussing something that Roman historians noted also.
I am not arguing against your skepticism, but we are talking about a period when there is a written recorded history.
Google "Christian cross history" and see what comes up.
The evidence is likely significant and quite convincing.
I just wonder how long it went on covertly . . . and how much of that evidence would have remained over the centuries.
Thanks for the link.