What does Romans 11:8 mean?
Paul follows his pattern of supporting his arguments with quotes from the Old Testament—the Jewish Scriptures. He has just written that God has hardened most Israelites in regard to faith in Christ.To show that God has always done this, even with Israel, Paul seems to assemble the quote in this verse from Isaiah 29:10 and Deuteronomy 29:4. In both instances, God was said to have kept Israel from understanding what was true. The people were overcome by a spirit of stupor, or sleepiness, and were not given eyes to see and ears to hear. God was the one who kept them from understanding.
Again, some might ask if this is fair on God's part. Paul made clear in Romans 9, however, that God retains the right to give or withhold understanding, to soften or harden hearts, as He sees fit and for His greater purposes. Just as Pharaoh was resistant to God, and so God enhanced that resistance, Israel's "hardening" comes as part of God's overall plan to fulfill His own purposes.