Not sure how you define "biggest" but WWII killed the most people and WWI is probably a close second and neither of those were primarily motivated by religion,

Not sure how you define "biggest" but WWII killed the most people and WWI is probably a close second and neither of those were primarily motivated by religion, but rather nationalism. I'd suggest you check out Tom Holland's "Dominion" if you'd like a well-researched and nuanced take on the effect of (Judeo-Christian) religion on Western civilization.