Why would anyone claim that what they know about afterlife is the ultimate truth?

I simply told them that maybe Scientology is right about reincarnation and that nobody really knows what actually happens after we die and they got offended 🤔
I was also given the outdated answer that this was mentioned in the quran and Islamic sunah but I know it isn't lol even if it's there it's not clear and if it's clear as day it's still just another point of view and we're back to square one.