What's A 'Belief System'?

‘Belief’ is defined as: “an acceptance that something exists or is true, especially without proof” or “something one accepts as true or real; a firmly held opinion.”

It is the idea of being ‘firmly held’ to a belief that defines its close minded nature.  To hold to a ‘belief’ closes our minds off to what may - if we were honest - undermine what we are determined to hold onto.  In comparison, it is not a ‘belief’ in and of itself to think - for example - that ‘I believe a deity may exist but I am on open minded as to finding out whether it really does or not’.