Peace to you, Zound!
Christ actually speaks of it Himself.
"... If you are the Christ, tell us plainly."
'Jesus' answered, "I did tell you, but you do not believe. The miracles I do in my Father's name speak for me, but you do not believe because you are not my sheep. My sheep listen to my voice; I know them and they follow me. I give them eternal life, and they shall never perish; no one can snatch them out of my hand. My Father who has given them to me, is greater than all; no once can snatch them out of my Father's hand. I and the Father are one."
Again the Jews picked up stones to stone him, but 'Jesus' said to them,
"I have shown you many great miracles from teh Father. For which of these do you stone me?"
"We are not stoning you for any of these," replied the Jews, "but for blasphemy..."
Despite the miracles that they saw, they did not believe because they were not his sheep. (this does not mean that anyone who rejected him could not turn and repent and come to Him another time, mind you, as some later did)
Some people will not believe even a miracle... because they do not want to believe the truth that goes WITH the miracle. Or face that they are wrong, or that they are following a lie... something the truth would reveal about them and their 'cherished' beliefs. Same reason some people start flinging ad hominem attacks... (they did say that my Lord had/was possessed by a demon, and that He cast out other demons in the name of Satan... )... rather than look at the truth of something being stated and/or staring them right in the face. They will harden their hearts against Him, and are no longer be ABLE to see or hear.
Consider that Israel began to grumble even after Moses showed them many miracles and freed them from Pharoah. And even after all that, Moses took 'too long' up on the mountain, and the people built themselves a golden calf, deciding that he would never return. Despite everything that they had seen and survived. People today often say, 'show us a miracle and we will believe'... and some might. But those who do not want to believe, who have hardened their hearts... these ones will find a way NOT to believe. Some of them would not even LOOK at the miracle (or truth).
Some are also so focused on the image they have built of God ... that they do not recognize and so cannot see the Truth of God sent to them.
Peace to you,