[Sethomas]

If you want to compare apples to apples, it's been exceptionally rare that any sizable group has been executed simply for having certain beliefs. However, religion is a convenient label in history to see where people set their alliances. You can't really rationalize the cooperation seen between select groups of Christian and Muslim mercenaries in the Crusades if each group mutually saw the other as subhuman and worthy of annihilation, as revisionists like to paint. However, it was very convenient to say "hey, this whole shebang was started by the Muslims, so let's go for Christian unity in a war about control over land mass."

If you want to talk about the Inquisition, especially the Spanish one, you had better have a great deal of research behind you because it's quite a complex issue. At any rate, the Church lacked the authority to condemn people to death, so the few tens of thousands that died in it did so at the hands of the state.

The same goes for the other side. For example, a family friend of mine is pure-bred Vietnamese, but his great-grandfather was executed for being Catholic. Were those responsible all that concerned about his religious beliefs? No, but to be Catholic implied a political bond or at least sympathy with the French, and that's why he lost his head.

And going back to apples to apples, there is nothing fundamentally antitheist about far-left political ideology. However, Marx and his followers made the brash assumption that religious affiliation would espouse a conflict of interest in a worker's uprising, and hence historical attempts at communism followed suit and executed indiscriminately for any allegiance other than to the state. So, it's absolutely idiotic to say "Christians killed because they're Christian, but atheists killed because, umm... they felt like it."