And he said, Go, and tell this people, Hear ye indeed, but understand not; and see ye indeed, but perceive not. Make the heart of this people fat, and make their ears heavy, and shut their eyes; lest they see with their eyes, and hear with their ears, and understand with their heart, and convert, and be healed.
Isaiah 6:9-10 (KJV)
The kingdom of darkness knows well that its number one enemy is the truth, the Gospel truth. So the enemy of truth has built a formidable defense against the Gospel, to shut it off from the human eyes, ears, and heart. To such a degree that even if they hear and see, yet they will not understand. Whenever this formidable barrier is penetrated and broken, the result will be conversion and healing.