Belief System

Is this a useful or even meaningful phrase? Considering how sacred or important belief itself is, it seems reasonable that thinking critically about anything having to do with belief is a good idea. Just look at that last sentence - it contains the words belief, thinking, and idea. It can’t be very straightforward precisely how these words and their associated concepts relate to one another. And how things relate to each other is what systems are all about, right? What is a belief system? What is a ‘what is...?’ question?