John 12:39

For this reason they could not believe, for Isaiah said again,

John 5:44

"How can you believe, when you receive glory from one another and you do not seek the glory that is from the one and only God?

Isaiah 6:9-10

He said, "Go, and tell this people: 'Keep on listening, but do not perceive; Keep on looking, but do not understand.'

Isaiah 44:18-20

They do not know, nor do they understand, for He has smeared over their eyes so that they cannot see and their hearts so that they cannot comprehend.

John 6:44

"No one can come to Me unless the Father who sent Me draws him; and I will raise him up on the last day.

John 10:38

but if I do them, though you do not believe Me, believe the works, so that you may know and understand that the Father is in Me, and I in the Father."

2 Peter 2:14

having eyes full of adultery that never cease from sin, enticing unstable souls, having a heart trained in greed, accursed children;

Treasury of Scripture Knowledge

Summary

Therefore they could not believe, because that Esaias said again,

Bible References

They

John 5:44
"How can you believe, when you receive glory from one another and you do not seek the glory that is from the one and only God?
John 6:44
"No one can come to Me unless the Father who sent Me draws him; and I will raise him up on the last day.
John 10:38
but if I do them, though you do not believe Me, believe the works, so that you may know and understand that the Father is in Me, and I in the Father."
Isaiah 44:18
They do not know, nor do they understand, for He has smeared over their eyes so that they cannot see and their hearts so that they cannot comprehend.
2 Peter 2:14
having eyes full of adultery that never cease from sin, enticing unstable souls, having a heart trained in greed, accursed children;

General references

Exodus 10:20
But the LORD hardened Pharaoh's heart, and he did not let the sons of Israel go.
Deuteronomy 29:4
"Yet to this day the LORD has not given you a heart to know, nor eyes to see, nor ears to hear.
John 1:5
The Light shines in the darkness, and the darkness did not comprehend it.
John 9:18
The Jews then did not believe it of him, that he had been blind and had received sight, until they called the parents of the very one who had received his sight,
John 10:37
"If I do not do the works of My Father, do not believe Me;