Jesus promised that those who ask, seek, and knock will find that for which they are searching (Matt. 7:7-8). However, in John 7:17, Jesus indicated another characteristic that is possessed by those who are able to know and understand the truth. "If anyone wills to do His will, he shall know concerning the doctrine, whether it is from God of whether I speak on My own authority."
Why is it that some people can read the Bible and never seem to get anything out of it, while others grasp its meaning and treasure its teachings for a lifetime? The answer is found in the motivation behind the reading. Only those who are intent upon performing God's will are going to put forth the effort necessary to determine the validity of what they are learning. The obtaining of life-changing truth is pursued much differently than the accumulation of meaningless trivia. If we have first determined that we are going to obey the Lord's will, nothing will stand in the way of us learning that will. The Lord's promise is that if we "will to do His will" we "will know."
James said that we are to be doers of the word, not hearers only who deceive themselves. The one who only hears the word will quickly forget what he has learned, but the one who intends to, and indeed does, what he has heard will be blessed in what he does (James 1:22-25).
The Bible can be understood by those who desire to understand it. Knowing the will of God is not the result of a work performed upon our hearts by the Holy Spirit (as some claim), but the result of our hard work in searching the Scriptures; hard work which springs forth from a heart that is intent upon obeying the Lord.