its a belief based on faith
Okay, I'll restate my question, although in the colloquial sense, religion is a beliefe based on faith.
Why makes people so sure their belief based on faith (religion) is the one true belief based on faith/religion?
Although, dictionary.com says otherwise:
re·li·gion
1.a set of beliefs concerning the cause, nature, and purpose of the universe, esp. when considered as the creation of a superhuman agency or agencies, usually involving devotional and ritual observances, and often containing a moral code governing the conduct of human affairs.
2.a specific fundamental set of beliefs and practices generally agreed upon by a number of persons or sects