John 12:40

God [of this world – Satan] has blinded their eyes and hardened their hearts so that they can neither see nor understand nor turn to me to heal them.”

Romans 9:18

18 Therefore, God has mercy on whom he wants to have mercy, and he hardens whom he wants to harden. — 32 Wherefore? Because they [Jews] sought it not by faith, but as it were by the works of the law. For they stumbled at that stumbling stone [Jesus Christ].

[Note: God has mercy on those who accept Jesus as their Saviour according to his Plan of Salvation.]

John 12:40

40 He [Satan] 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 [God] should heal them.

Romans 11:25

I do not want you to be ignorant of this mystery, brothers and sisters, so that you may not be conceited: Israel has experienced a hardening in part until the full number of the Gentiles has come in,

[Note: refers to Believers whether Gentile or Jewish can lose their salvation if they do not continue to trust and have faith in Jesus according to God’s Plan of Salvation. Some Jews are saved now but most are not.  If Jews are alive in the flesh at the 2nd coming they will have the opportunity to accept Jesus as their Saviour and will do so.  Those Jews or Gentiles who die unsaved are lost forever.  They get cut off from the vine.]

2 Corinthians 3:14

14 But their minds were blinded: for until this day remained the same vail untaken away in the reading of the old testament; which vail is done away in Christ.

[Note: blinded, hardened, and dulled because they rejected Jesus. Accepting Jesus would have removed the vail.]

Ephesians 4:17-19

17 This I say therefore, and testify in the Lord, that ye henceforth walk not as other Gentiles walk, in the vanity of their mind,

18 Having the understanding darkened, being alienated from the life of God through the ignorance that is in them, because of the blindness of their heart:

19 Who being past feeling have given themselves over unto lasciviousness, to work all uncleanness with greediness.

Hebrews 3:13

13 But exhort one another daily, while it is called Today; lest any of you be hardened through the deceitfulness of sin.

2 Corinthians 4:4 [Full Chapter]

4 In whom the god [Satan] 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.

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Conclusion:

Satan and/or individual choice causes hardening, blindness, and dullness toward God.  God allowed but did not cause hardening, blindness, or dullness of anyone’s heart.