One can believe every statement that Christ makes, just as he is able to believe any other person. We should not tell men they cannot believe. It lies deep in man's nature. Through the fall and our own sin, the nature of man has become so debased, depraved, and corrupt, that it is impossible for him to come to Christ without the powerful work of God the Holy Spirit. We see it in the animal world.
Animals act according to their nature.
Sheep will not eat garbage, and a hog will not eat grass. The reason sheep will not eat garbage is because of their nature. Man's nature keeps him from coming to Christ. Give a mother a knife and tell her to plunge it into her baby. The mother's nature makes it impossible.