Truth is in the eye of the beholder when it comes to things beleived. Meaning that no matter what the facts/proof is/are, one can still choose to believe whatever they want. For example: 1+1=2 is a fact - put 1 apple with another apple and they become a group of 2 apples. But, if one wants, they can believe 1+1=56312. Facts/evidence shows this to be untrue, but it doesn't matter to the one who wants to believe what they want to believe. Basically, humanity doesn't need facts when it comes to believing something true or not. How many of us know someone that lies so much that they eventually believe what they are saying? Similar concept.How can one teach that which one did not experience? One who realizes the supreme truth cannot explain about it and one who did not realize goes about explaining the so called 'truth'.
So in that regard, anyone can teach anyone what they believe to be true (regardless of if it's really true or not, based upon the facts), but it's up to the one being taught if they want to take facts, or opinions/beliefs and make up their own mind.
The counterpoint is that what is factually true, can be taught if it hasn't been experienced if what's being taught doesn't require personal experience. Again though, we are back to believing what is true and what isn't true.
Maybe one reason why people have this problem is the fact that they consciously make "truth" and "belief" to be the same, when they are very different. Maybe this helps them justify their beliefs....?