Like I said, it's complicated.

My ancient ancestors were very devout in their religion and did believe in an afterlife. That's been largely abandoned in recent times with some exceptions. These days, most Mandalorians believe in the manda. Which is like a collection of souls that are the essence of what it means to be Mandalorian. A Mandalorian ignorant of their culture and heritage is considered to be soulless, and would have no part in the manda. This is seen as a disgrace.