35 So Jesus said to them, “ For a little while longer the Light is among you. Walk while you have the Light, so that darkness will not overtake you; he who walks in the darkness does not know where he goes. 36 While you have the Light, believe in the Light, so that you may become sons of Light.” These things Jesus spoke, and He went away and hid Himself from them. 37 But though He had performed so many signs before them, yet they were not believing in Him. 38 This was to fulfill the word of Isaiah the prophet which he spoke:“ Lord, who has believed our report? And to whom has the arm of the Lord been revealed?” 39 For this reason they could not believe, for Isaiah said again, 40 “HE has blinded their eyes and HE hardened their heart, so that they would not see with their eyes and perceive with their heart, and be converted and heal them.” John 12:35-40 NASB
8 Then I heard the voice of the Lord, saying, “Whom shall I send, and who will go for Us?” Then I said, “Here am I. Send me!” 9 He said, “Go, and tell this people: ‘Keep on listening, but do not perceive; Keep on looking, but do not understand. ’ 10 “Render the hearts of this people insensitive, Their ears dull, And their eyes dim, Otherwise they might see with their eyes, Hear with their ears, Understand with their hearts, And return and be healed.” Isaiah 6:8-10 NASB
This is the passage that John quoted in John 12:40, which is in context at the top of this post. Notice v39, “For this reason they could not believe…” This is the NASB rendering. The NA28 Greek text for this part of v39 is, “διὰ τοῦτο οὐκ ἠδύναντο πιστεύειν.” Here is my rendering, “Through this they were not able to believe.” The word translated here as “not” is οὐκ or ouk, which is an absolute negative. Therefore what it is defining is absolutely negated and that verb is ἠδύναντο, which is the 3rd person, imperfect tense, indicative mood, middle voice form of δύναμαι or dunamai, which means “to be able, either intrinsically and absolutely, which the ordinary signification; or, for specific reasons.” The third part of this is the word “believe” which is a translation of the verb πιστεύειν the present tense, infinitive mood, active voice form of πιστεύω or pisteuō, “to have faith.” The way this is structured my brethren it is plainly a work of God that these people were not able “to have faith.”