This is where things get complicated (as is often the way with religion). You don’t just have to describe religious practices. You have to get into the characters’ heads and work out why beliefs make them act and talk as they do.
In this article, Amanda McCrina claims that the vast majority of historical fiction mostly ignores religious practices and the pervasiveness of religion in people’s lives, making them more modern in outlook than they really were. This makes life easier for the modern writer and also makes it easier for the modern reader to identify with the characters.