Hardened Against God
They are the Israel within Israel (9:6), the “remnant chosen by grace” (11:5). These Jews, “the elect” (11:7), have obtained the right standing with God that Israel as a whole was seeking.
God confirms the spiritual insensitivity that people are locked up under by virtue of their sin in Adam.
The quotation in verse 8 is a composite. Most of it comes from Deuteronomy 29:4, but the phrase “spirit of stupor” is from Isaiah 29:10
He spells this out with references to eyes that do not see and ears that do not hear. That part of them that ought to be most sensitive to the divine leading is not operative, not functioning as it was intended to function.
Perhaps Paul is thinking that the Scriptures, which should have been a source of spiritual nourishment to the Jews, are in fact the very thing that brings about their downfall, for they use them wrongly.