John 8:27

but they did not perceive that he meant the father.

John 8:43

why do not you understand my language? unless because ye cannot bear my doctrine?

John 8:47

he that is of God, does therefore observe what God says: which you do not observe, because ye are not of God.

Romans 11:7-10

"What then, hath not Israel obtained what he sought for?" no, but the chosen remnant have obtained it, and the rest are obdurate to this day,

2 Corinthians 4:3-4

but if our gospel be obscure, it is so only to those who are lost:

Treasury of Scripture Knowledge

Summary

They understood not that he spake to them of the Father.

Bible References

General references

John 8:43
why do not you understand my language? unless because ye cannot bear my doctrine?
Romans 11:7
"What then, hath not Israel obtained what he sought for?" no, but the chosen remnant have obtained it, and the rest are obdurate to this day,
2 Corinthians 4:3
but if our gospel be obscure, it is so only to those who are lost: