I started to upvote - and then I wondered if that was actually true.
I believe the internal KGB figures were just under 40 million for Stalin, and if you ascribe say 60 million to Mao, the jump to beat is then about 100 million. These are generous numbers that take in the effects of willful starvations and the like.
The Crusades are sort of low at just 1-3 million, but they are only a small element on the counterbalance. For example it'd probably be fair to say Spanish colonial wars in the Americas were specifically guided heavily by the Roman Catholic ideology of saving the heathen (hmmm...), with conquest rationalized to fund it, and Wikipedia cites estimates running from 15 to 70,000,000 with a geometric mean of 34 million. And that's before we weight for the religious dimension of purportedly "secular" conflicts, let alone begin to count the religious wars elsewhere thoughout the non-Eurocentric world such as China, India, and Africa. They are quite a bit more than is generally known.
And once the numbers are leveled out better, the savagery is no less lacking, and in fact probably much greater, on the religious side. Random cite, Catholics in France I think slaughtered 100,000 Protestants on just the St Bartholomew's Day massacre (a modern Order 66 so to speak), and for a more qualitative example in another instance,
“Our men spared no one, irrespective of rank, sex or age, and put to the sword almost 20,000 people. After this great slaughter the whole city was despoiled and burnt, as Divine vengeance raged miraculously…” (“The History of the Albigensian Crusade”, Tr. W.A. Sibly & M.D. Sibly)
That was of course the war that famously gave us Rome's words, "Kill them all [ie. friend and "foe" alike] - God will know his own."
So both those two probably still beat out any of the popes individually, but history is long and wide, and just gauging by feel for the basic statistics, absent further looking, I'm not even close to sure your general statement is actually true and suspect it may more likely be false.