That which is directly experienced, and that which does the experiencing. Neither can be captured or expressed completely in concepts (thoughts or words).
What about belief?
Beliefs are just thoughts. Whether or not they are believed is unimportant.
What is important?
Nothing. But it seems helpful to distinguish between what can be known and what cannot so the search for "Truth" can relax.
What is Truth?
"Truth" is another concept. It's irrelevant.
What is relevent?
The current, ongoing flow of experience. That's all that's relevant because that's all that can be known.
What about the Bible, the Koran, the Gitas, the Vedas, the Sutras, all the sacred texts? Don't they merit belief?
They appear as part of the flow of experience. To the extent that they make claims about things that are beyond experience, they can't be confirmed or debunked within experience. Until their claims are confirmed or debunked within experience, all that can be said is, "That may or may not be true. I have no way of knowing."