Most beliefs oppose the truth in one way or another. Some much moreso. The ego is in complete opposition to the truth, at its deepest levels.
So it's not really a matter of who is right or wrong, but moreso just a matter of how 'opposing' a person is being. How much they are trying to reject truth and reality.
If you can't accept reality, or what is, or real truth, or love, or God, then you are in opposition, no matter what form it takes.
The ONLY way that you can be happy and at peace, then, is to learn to ACCEPT THE TRUTH of what is, rather than oppose it.
You cannot be opposed to the truth and be at peace. You cannot be opposed to the truth and feel happy and fulfilled. You cannot be opposed to the truth and feel loved. Because the truth IS love.