they were unable to believe: John quotes Isaiah 6:10 to explain Israel's lack of belief (see Matthew 13:13–15; Mark 4:12; Luke 8:10; Acts 28:26–27). When God's revelation comes, we must believe. If we refuse, the light disappears (John 12:35–36). When God’s light departs, the darkness closes over unbelieving hearts. This means that continued rejection leads to deeper spiritual blindness.