"And when He has come, He will convict the world of sin, and of righteousness, and of judgment:  "of sin, because they do not believe in Me
To the pure all things are pure, but to those who are defiled and unbelieving nothing is pure; but even their mind and conscience are defiled.
"But you do not have His word abiding in you, because whom He sent, Him you do not believe.
But without faith it is impossible to please Him, for he who comes to God must believe that He is, and that He is a rewarder of those who diligently seek Him.
But when some were hardened and did not believe, but spoke evil of the Way before the multitude, he departed from them and withdrew the disciples
Nevertheless they would not hear, but stiffened their necks, like the necks of their fathers, who did not believe in the LORD their God.
2 Kings 17:14
"He who believes in Him is not condemned; but he who does not believe is condemned already, because he has not believed in the name of the only begotten Son of God."
"He who believes in the Son has everlasting life; and he who does not believe the Son shall not see life, but the wrath of God abides on him."
"Therefore I said to you that you will die in your sins; for if you do not believe that I am He, you will die in your sins."
So we see that they could not enter in because of unbelief.
that they all may be condemned who did not believe the truth but had pleasure in unrighteousness.
2 Thessalonians 2:12
But I want to remind you, though you once knew this, that the Lord, having saved the people out of the land of Egypt, afterward destroyed those who did not believe.
"But the cowardly, unbelieving, abominable, murderers, sexually immoral, sorcerers, idolaters, and all liars shall have their part in the lake which burns with fire and brimstone, which is the second death."
Do not be unequally yoked together with unbelievers. For what fellowship has righteousness with lawlessness? And what communion has light with darkness?
2 Corinthians 6:14
Beware, brethren, lest there be in any of you an evil heart of unbelief in departing from the living God
But even if our gospel is veiled, it is veiled to those who are perishing,  whose minds the god of this age has blinded, who do not believe, lest the light of the gospel of the glory of Christ, who is the image of God, should shine on them.
2 Corinthians 4:3-4
"How can you believe, who receive honor from one another, and do not seek the honor that comes from the only God?
Harden not your heart, as in the provocation, and as in the day of temptation in the wilderness: When your fathers tempted me, proved me, and saw my work. Forty years long was I grieved with this generation, and said, It is a people that do err in their heart, and they have not known my ways: Unto whom I sware in my wrath that they should not enter into my rest.
Unto you therefore which believe he is precious: but unto them which be disobedient, the stone which the builders disallowed, the same is made the head of the corner, And a stone of stumbling, and a rock of offence, even to them which stumble at the word, being disobedient: whereunto also they were appointed.
1 Peter 2:7-8
But let him ask in faith, with no doubting, for he who doubts is like a wave of the sea driven and tossed by the wind.  For let not that man suppose that he will receive anything from the Lord
"You search the Scriptures, for in them you think you have eternal life; and these are they which testify of Me.  "But you are not willing to come to Me that you may have life.
"But he said to him, 'If they do not hear Moses and the prophets, neither will they be persuaded though one rise from the dead.' "
The fool has said in his heart, "[There is] no God." They are corrupt, and have done abominable iniquity