Non-believers are incapable of understanding the Bible, as God has not opened their eyes.
12 Now we have received, not the spirit of the world, but the spirit which is of God; that we might know the things that are freely given to us of God.
13 Which things also we speak, not in the words which man’s wisdom teacheth, but which the Holy Ghost teacheth; comparing spiritual things with spiritual.
14 But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned.
Satan blinds those that do not believe, so as to ensure they never know the truth. Little “g” god, here, denotes Satan.
2 Corinthians 4:4-6 KJV
4 In whom the god of this world hath blinded the minds of them which believe not, lest the light of the glorious gospel of Christ, who is the image of God, should shine unto them.
5 For we preach not ourselves, but Christ Jesus the Lord; and ourselves your servants for Jesus’ sake.
6 For God, who commanded the light to shine out of darkness, hath shined in our hearts, to give the light of the knowledge of the glory of God in the face of Jesus Christ.
And of course, there’s the willfully ignorant:
2 Peter 3:3-7 KJV
3 Knowing this first, that there shall come in the last days scoffers, walking after their own lusts,
4 And saying, Where is the promise of his coming? for since the fathers fell asleep, all things continue as they were from the beginning of the creation.
5 For this they willingly are ignorant of, that by the word of God the heavens were of old, and the earth standing out of the water and in the water:
6 Whereby the world that then was, being overflowed with water, perished:
7 But the heavens and the earth, which are now, by the same word are kept in store, reserved unto fire against the day of judgment and perdition of ungodly men.
Collection of Verses by Nikki VanDuyn