I heard that Andre wouldn't let an atheist friend of his (ollllld friend) hold his child. Andre is very devout, I won't even fathom mentioning that I'm not religious to him.
Based on no real evidence but anecdotes and assumption, it's cultural. Brazil is rising in world status, but for the most part life is harder there. It's freaking easy to be an atheist in lower middle class America, where shit is just good enough in this life. For many (not all) BJJ practitioners in Brazil, they come from lower class homes. I don't need to describe in detail, but many aspects of many of my friends lives are scary. They need to believe in God to get them through the day; they need to believe that something better is waiting for them because of the fact that they didn't luck out this life.
Interestingly enough, the IBJJF was pondering (if they didn't enforce) banning all patches with religious symbols. I think words are allowed (Team Jesus!) but they don't want crosses or anything. They said they've got a multitude of practitioners from a variety of religions, there's no point to create an unnecessary rift.