Romans 11:8

According as it is written, God gave them a spirit of stupor, eyes that they should not see, and ears that they should not hear, unto this very day.

Paul here quoted Deuteronomy 29:4, which reads,

Yet the Lord hath not given you a heart to perceive, and eyes to see, and ears to hear, unto this day.

This was spoken to a generation that had witnessed the miracles of God through Moses in their fantastic deliverance from Egypt through the Red Sea. The thrust of the words here is that although they had indeed "seen" such wonders, in the sense of stimuli on the retina of the eye, they had not grasped the true meaning and significance. This was appropriate and applicable to Paul's generation who had witnessed even the greater wonders of Christ but had somehow failed to get the message. The great realities are morally and spiritually understood. Thus, when Jesus condemned unbelief, he made it the consequence of moral blame rather than of intellectual doubt (John 3:19).

There was doubtless another point in Paul's introduction of this passage from Deuteronomy describing the lost generation of the wilderness. They themselves were another outstanding historical example of God's judicial hardening and destruction. Due to the promise of the Messiah, God did not destroy them, but delayed their entry into Canaan until the death of the whole generation!