"Spiritual darkness is spiritual bondage. Those who rebel against the light of God's Word are punished with darkness, for they loved it and chose it. Let us dread the consequences of sin; if three days' darkness was so dreadful, what will everlasting darkness be?" (paraphrased from Matthew Henry's Commentary)
Though the ones ensnared may be surrounded by believers, just as the Egyptians were in community with the Israelites, they are seemingly not affected or pierced by the Light within them. They are ignorant to the truth because they are bent on sin. And those bent on sin have their backs to God. They might put up a good front in the presence of believing friends because such sin is often kept secret. The one stuck in sin is not always blatant about it. But no one can serve two masters. Our God is a jealous God and He will not compete. He will bring judgement.
So judgement came in response to Pharoh's hardened heart. "When God made this difference between the Israelites and the Egyptians, who would not have preferred the poorest cottage of an Israelite to the finest palace of an Egyptian? There is still a real difference, though not so discernible a one, between the house of the wicked, which is under a curse, and the habitation of the just, which is blessed.