That is all.
No, seriously, that’s it.
You don’t need more explaining.
RIP is just an expression wishing the dearly departed an unperturbed existence in the afterlife. Sure, it comes from Christian theology, but then so is Goodbye — short for God Be With You. Pretty much everyone in all other religions have said Goodbye if they’re fluent in English, so why not RIP? The technicalities don’t matter; what does is the sentiment.
We use countless words and terms that are grounded in past religions and beliefs including many that have been lost to history.
I’m a Buddhist/quasi-Atheist and that hasn’t stopped me from saying Merry Christmas either or Godspeed if I’m wishing a safe journey. Many in the scientific world use Godspeed, especially toward astronauts or those about to conduct high altitude research and quite a bunch of them are non-religious as well.
But I would get my panties in a bunch if the phrase became mandatory, which last I checked, it wasn’t. Shall we stop walking on eggshells already?
This is a bit of an odd post, but I was prompted to write it because I saw that question in the stats as one of the search terms linking to this blog. I guess it’s been a while since I wrote anything about culture (also, I’ve seen this question asked elsewhere) so might as well put it to rest here.