Belief means you are mostly convinced but not completely sure. Knowledge is more certain. These two words can be used as follows—I believe the universe probably started with the Big Bang, but I know the earth revolves around the sun.
We derive a degree of intellectual well-being from the beliefs and knowledge afforded us by modern science and engineering. But knowing that the earth revolves around the sun or that the Big Bang is the most likely explanation we now have does not provide us with very much emotional or psychological well-being.
We need more, or we need something different, to achieve a deep state of emotional well-being.
To achieve a deep state of emotional well-being we need to know that we can really believe at least one other person. That person should be our primary interlocutor, the person we deal with the most. If we cannot believe that person and/or they cannot believe us, we can’t achieve a deep state of emotional well-being.
To the best of my knowledge, there exists no common communication system (or even uncommon one) that allows us to deeply know and believe someone else, except the FIML system.
The reason this is so is all other human communication systems rely too much on implication, interpretation, and assumed shared beliefs.
When you do any of those things with your primary interlocutor you will necessarily make mistakes and/or be uncertain about what they are saying or how they are understanding you. Mistakes and uncertainty create shadowy feelings and wrong or multiple interpretations in the mind. Rather than have a clear knowledge of what your partner thinks or understands, you will be guessing.
Even if you are right every time you guess (and this is not possible), you will still have no way of being certain. You will not know if you are right or not. And your partner will have the same problem with you.
Emotional well-being depends on the quality of our communication with our primary interlocutor. There are substitutes—careers, religions, political causes, money, power, sex, etc.—but none of these will ever equal the emotional well-being that comes from very high-quality communication with your primary interlocutor. To have high-quality communication, you and your partner must have a system that removes doubt and uncertainty and replaces them with knowledge and belief.