It's my belief that the thief is somebody known to us.
noun Faith or trust in the reality of something; often based upon one's own reasoning, trust in a claim, desire of actuality, and/or evidence considered.
My belief is that there is a bear in the woods. Bill said he saw one.
noun Something believed.
The ancient people have a belief in many deities.
noun The quality or state of believing.
My belief that it will rain tomorrow is strong.
noun Religious faith.
She often said it was her belief that carried her through the hard times.
noun One's religious or moral convictions.
I can't do that. It's against my beliefs.
noun ↑ Jespersen, Otto (1942), A Modern English Grammar on Historical Principles (Sammlung germanischer Elementar- und Handbücher; 9), volume VI: Morphology, London: George Allen & Unwin, published 1961, § 12.23, page 200.