John 8:43 KJV
Why do ye not understand my speech? even because ye cannot hear my word.
Since the Bible must be spiritually discerned Christians have come to expect that unsaved people (and sometimes saved people who have entangled themselves in worldliness) just cannot understand the Bible. Jesus expressed it in this verse. They could not understand the style of His speech, His use of metaphors and symbolism, because they had no grasp of spiritual truth.
Cambridge study notes says,
"They perpetually misunderstand His language, because they cannot appreciate His meaning….
They ‘cannot hear;’ it is a moral impossibility: they have their whole character to change before they can understand spiritual truths."
This explains why a Christian can plainly express the Word of God and yet still the person with whom he speaks does not get it. It cannot be gotten unless it is gotten from the character of the Spirit of God. The Bible says faith is a gift of God. It cannot be worked up in a man but it can be passed on from one who has faith. This is why it is so important for Christians to be vocal about their faith. It is how God moves in others to see and understand the truth of His Word.