You're right about Gustavus Adophus. The 30-years war started as a religious war but later changed into a more politically oriented conflict as varous groups changed sides. The contemporary estimates of 26 million dead seem too high. There may have been 26 million displaced, which is probably where the estimates came from. Many villages were erased during the war, but the populace may have just moved.
A more religiously oriented war would be the struggles under Elizabeth I. Before there were Catholics and Protestants in England. Later there were English who happened to be Catholic and English who happened to be Protestant. The Cromwell reign was also religious in nature includeing the destruction of much of the religious music of England.
I love the restriction of range fallacy by which modern freethinkers and secularists try to stain Christianity and Judaisms with blood while trying to keep their own movement free from the taint of mass-murder.
Seeing that modern atheism arose as a reaction against the wars of religion in Western Europe after the collapse of confessional unity within the Patriarchate of Rome, it, as an indentifiable movement, is about 400 years old. I propose considering a body count for the first 400 years of each movement.
Let's see:
- The Christian Church. Let's be generous to the other side, and stretch it out to 500 years so we can count the riots by the Nitrian monks in Alexandria against our side, a few hundred dead.
- Judaism. It rather depends on where you start counting: If we start with Abraham, I think you get a few, say hundreds, dead in battle. If with Moses, rather bloodier with the conquest of Canaan, but still surely dead in the tens of thousands only. If with rabbinic Judaism after the fall of the Temple, I think you would need to investigate the degree to which Judaism and a religio licta in the Roman Empire participated in the persecution of the Church. Probably a few hundred dead tops.
- Modern atheism. Hundereds of millions dead. We can count not only the dead of Soviet, Chinese and Cambodian communism, but the tens of millions dead from "choice"--abortion on demand. (Don't try to squirm out on that one--they are biological humans, members of our species.)
- Islam. Even Islam which started out as a bloody religion of conquest did not manage to kill that many in its first 400 years.
Now, it may be objected that the population of the world is larger now. Ok, put in a scale factor of 100 against the monotheistic religions, and secularist atheism still out-kills by a large margin.
I shudder to think what the death-toll from secularist atheism will look like by the time it is as old as Christianity was when the 30-Years war ended.