Romans 11:8 (According as it is written, God hath given them the spirit of slumber, eyes that they should not see, and ears that they should not hear;) unto this day.
The context of the word "them" and "they" in Romans 11:8 is that of national Israel. Right now, they are an unbelieving people because of the "spirit of slumber."
The Bible says in John 12:39-40
Therefore they could not believe, because that Esaias said again, He hath blinded their eyes, and hardened their heart; that they should not see with their eyes, nor understand with their heart, and be converted, and I should heal them.
A sure fire way to becoming deaf and blind is unbelief. It will dull your hearing, give you blurry vision, and harden your heart.
But remember, 2 Corinthians 4:4 gives us the cure for spiritual blindness and spiritual heart disease:
2 Cor 4:4 In whom the god of this world hath blinded the minds of them which believe not, lest the light of the glorious gospel of Christ, who is the image of God, should shine unto them.
Begin your day by shining the light of the gospel.