> I also believe you have to follow the Greek Orthodox religion to be able to visit - they might make some rare exceptions.

You need a permit (effectively, a visa) to enter the peninsula, and there is a daily quota. About a tenth of available permits can be issued to non-Orthodox visitors. They don't single out the Greek church from other Orthodox churches (AFAIK).

[I researched the entry requirements a couple of years ago out of curiosity, but never tried to actually visit. Something may have changed in the meantime, but somehow I doubt it :)]

Greek Orthodox religion is somewhat of a misnomer. There are other Eastern Orthodox churches which are in communion with the church in Greece. The religion is Eastern Orthodox, Greek is a language/hierarchy within orthodoxy.

Also, I'm reading a book about Mt Athos now, it was mentioned that a tourist was on the island. That must have taken place ~100 years ago, mind you.

No, that's not correct. People from other religions are allowed. It's just that there's a daily limit to the amount of people who can enter the region and the analogy is like 10:1 orthodox/non-orthodox. I think it's like 120 people in total, or something like that.

I personally know a Catholic who had no trouble getting a permit to visit, so I don't think that's true.