I believe it will never be known for sure (publicly). If the French government found undeniable proof of sabotage, I doubt they would ever admit it. Paris is already having massive weekly demonstrations, without pouring gasoline on the chaos.

I have nothing but suspicions. Who really knows. Odd that can churches in France burned down in a month though

Can you add other options? "Accidental fire during renovations" (which I don't believe, but still should be an option) and "We will never know due to lack of proper investigations" (or something along those lines).