Labarum refers to a military standard used in the Roman Empire that was adapted by the Emperor Constantine I as a symbol of Christianity. He combined the first two Greek letters of the word "Christ" (Greek: ΧΡΙΣΤΟΣ, or Χριστός)—Chi (χ) and Rho (ρ), and ordered his soldiers to place this symbol on their shields before a decisive battle. Constantine's subsequent victory was attributed to Christ's support for displaying the Labarum.
https://www.newworldencyclopedia.org/entry/Labarum
But it seems that the X was the emblem of a celtic god (a kind of Apollos) adored in Grand.
https://en.wikipedia.org/wiki/Archaeolo ... e_of_Grand
Constantin had shown himself as this celtic god redivivus, to surprise the celtic people. Hence he used this emblem.
But X is the Latin number 10. A prophecy said that Constantin will rule for 30 years, hence he had introduced the number 30, XXX in the his Labarum.
The reader can imagine easily how that celtic symbol was christianized after the battle on the bridge Milvius.
What is interesting is that in this case an Atwill would be right to suspect the Roman authority behind the use of a local religious symbol or label.