Refer to Isiah 29: 10 - 14
When we don't read God's word -- when we don't regard it as important enough to spend some time at -- God could make us dull as he has done in these Isaiah verses. When we rely too much on what men teach (v. 13 "their fear toward me is taught by the precept of men") we stupefy ourselves to what God has written to us. We find it easy to say, "we can't understand the Bible, it is a 'book that is sealed' We will not put in the effort".
Some, a few, passages are very difficult. Some we understand only after years or reading and thinking about them. Some are never more than partially understood. Some we never understand.
However, the actual number of verses in the Bible that one can not understand is very small. Almost all of them we can understand. Most all of them we can understand in English; we don't need to know Hebrew and Greek. We can understand them in Old English or New English or Slang English -- it's just not that hard.
There is one guarantee: if we don't read we will not understand. If we let someone else tell us what it means we only know that that person understands. It may be correct, or just useful, or just plain wrong.
When men go long enough not making the effort to read the Bible the wisdom and knowledge vanish. The very "wise" ones they depend on to tell them what God says -- or even to tell them what is "right" -- loose their wisdom.
