Well, it was Elen who started this thread, and all threads do wander off-topic eventually, so I'll leave it up to her to decide. Splitting the thread off is definitely possible.
There's a certain religion-agnostic aspect to churches as well. In the Highlander universe, churches, dojos and burial grounds are all considered Holy Ground, regardless of religion or creed. Holy Ground is the only place where Immortals can safely meet one another, because none of them — not even the most evil among them, as stated by Duncan MacLeod in the Highlander television series — will violate that law.
In the original movie, it is stated by Ramirez that "it is tradition", but in Highlander III: The Sorcerer and in the series, it is established that there will be some "magical intervention" if two immortals were to fight on Holy Ground. In Highlander III: The Sorcerer, Kane initially attacks Connor MacLeod in a dojo, but as his sword shatters that of Connor in an attempt to behead him, a mysterious wind flares up inside the dojo and causes a fire, and in the television series, it was hinted that the volcanic eruption at Pompeii was the result of two Immortals having a battle on Holy Ground.
It is my personal take that holiness is not tied in with religion, but rather with spiritual purity and sacredness.