All of our past experience and knowledge guides us as we move through life. After a time, we come to have faith in our knowledge, we start to behave based on that faith. When we are confronted with radical new knowledge and/or experiences, our faith is sometimes slow to change. Plato's Allegory of the Cave and the Gospel of Mary are both examples of people trying to impart their newfound knowledge onto others. In both cases it is difficult to do, and for many of the same reasons.
In Plato's Allegory of the Cave we are presented with a story of a group of people who are kept imprisoned in a cave for their entire lives, "Here they have been since childhood, chained by the leg and also by the neck, so that they cannot move and can see only what is in front of them, because the chains will not let them turn their heads" (p.227, the Allegory of the Cave). These people experience only what is presented to them in the form of shadows on the wall caused by the light of a fire that they perceive to be the sun. This is their world. The voices that they hear from the people controlling the shadows, they believe to be the voices of the shadows. This is the only world that they know and they have no reason to believe that there could be anything beyond it. .
In the story, one man escapes from the cave to find that everything that they have been witnessing in the cave is really just a representation of the real world, that the light from the fire is representative of the sun and so forth. When the man returns he attempts to explain to those still in the cave what is outside but they are unable to believe him, for it is so far beyond anything they have ever experienced, "What do you think he would say, if someone told him that what he had formerly seen was meaningless illusion, but now, being somewhat nearer to reality and turned towards more real objects, he was getting a truer view? Suppose further that he were shown the various objects being carried by and were made to say, in reply to questions, what each of them was.