John answered and said, A man can receive nothing, except it be given him from heaven. John 3:27
One of the last talks my friend and I had shortly before he died, was about how some could understand a doctrine while others could not. We concluded that there are some truths that cannot be conveyed unless the hearer is given the ability to understand. Not "has the capacity", but is given the gift of understanding.
My friend Walt was so gifted. Scriptures reached him easily and he understood.
I have had the sad experience of watching as people select out Bible verses that seem to support their beliefs, and reject others that conflict with them. You can look online at Bible commentaries that come to opposite conclusions using the same Bible. Then it hits you. People form their beliefs first, then interpret the Word accordingly. You find that in many cases you are not having a disagreement over the Bible at all. Something else is at work.
It could be experience, custom, or culture that helps to form the Great Enemy of Bible understanding--the Preexisting Belief. This built-in belief is often impossible to alter. "I still say" will win the day.
Here is where the gift of God is evident. When someone accepts a truth that totally contradicts a once strongly held belief, it is the Spirit of God at work.
Laurens Van der Post, in The Dark Eye in Africa, told of a tree that blocked a highway building project. The tree could not be budged, even when pulled by teams of oxen. A medicine man said he knew the answer. He made offerings to the demon that held the tree in place. Once he did this it was easily moved.
The doctrines of demons hold many peoples' minds stubbornly in place. Only by the grace of God can they be gifted to understand. No amount of evidence will convince them, and God does not give this gift of understanding to all.