I’ve been thinking about this for a minute, and I think a good standard here is making a list of (relatively) non-overlapping causes of death that have claimed over a billion human lives.
Infectious disease is almost certainly at least one entry on this list, primarily secular war as well, starvation/famine probably a few times over, cancer and heart disease are probably distinct entries, and death attempting to grow/hunt food. I suspect deaths by religion could be on that list as well, but it’s the entry I’m least confident in.
In every sense of the word, this is a bad list to be on, but I don’t think religion is near the biggest culprit on the list, even if you do a lot of special pleading, and group all deaths by religious cause together, but split each disease, war, etc up for some reason.
I will not stand for this gorgonzola slander