ἀπιστία

And He did not do many miracles there, because of their unbelief.

And He was amazed at their unbelief. And He went around from village to village, teaching the people.

Immediately the boy’s father cried out, “I do believe; help my unbelief!”

Later, as they were eating, Jesus appeared to the Eleven and rebuked them for their unbelief and hardness of heart, because they did not believe those who had seen Him after He had risen.

What if some did not have faith? Will their lack of faith nullify God’s faithfulness?

Yet he did not waver through disbelief in the promise of God, but was strengthened in his faith and gave glory to God,

That is correct: They were broken off because of unbelief, but you stand by faith. Do not be arrogant, but be afraid.

And if they do not persist in unbelief, they will be grafted in, for God is able to graft them in again.

I was formerly a blasphemer, a persecutor, and a violent man; yet because I had acted in ignorance and unbelief, I was shown mercy.

See to it, brothers, that none of you has a wicked heart of unbelief that turns away from the living God.

So we see that it was because of their unbelief that they were unable to enter.