John 12:39

39For this reason they could not believe,
The author explicitly states here that Jesus’ Jewish opponents could not believe, and quotes Isa 6:10 to show that God had in fact blinded their eyes and hardened their heart. This OT passage was used elsewhere in the NT to explain Jewish unbelief: Paul’s final words in Acts (28:26–27) are a quotation of this same passage, which he uses to explain why the Jewish people have not accepted the gospel he has preached. A similar passage (Isa 29:10) is quoted in a similar context in Rom 11:8.
because again Isaiah said,

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