Belief is an Indefinite Word
We can define some words in terms of other words.
For example, I can say that a tabby cat is a domestic cat with a distinctive coat patterns such as stripes, dots, lines, or swirling patterns. If you then see a cat with these pattens, you can make an argument that it qualifies as a tabby.
The problem is that no one really knows what a belief is.
Is belief a human emotion, similar to happiness? Is it consensus across all parts of the brain? Is there a brain region for belief? Does it mean provability?
To one person, if I say “I believe in God,” that might indicate to them that I pray and look to religious texts for wisdom, whereas to another person it might indicate that I have an argument for Gods existence, and therefore that I could convince someone else to accept his existence by words alone.
Saying “I believe X” should be done with hesitation, as it is easy to create more misunderstanding than understanding. When such misunderstanding is found in the listener or reader, care should be made to bring it back to more basal facts: “I heard this in a Podcast, I read this in a magazine, researchers claim to have done X experiment which produced Y results, astronomers observed Z last week.”