DA Carson, in his commentary on the gospel of John, focuses on four things regarding God hardening people's heart. "God's sovereignty in these matters is never pitted [fights] against human responsibility."
The Bible often puts God's sovereignty and human responsibility side by side. (See Acts 4:27-28.)
And in Acts 2:23, we see that God the Father predestined Christ to die on the cross yet held the people accountable for killing Jesus. "God's judicial hardening is not … the capricious manipulation of an arbitrary Sovereign God toward morally neutral or good people, but rather His holy condemnation of guilty people who are condemned to the judgment that they themselves have chosen."
There are many examples of this in the Bible (i.e.: Deuteronomy 2:30; 1 Samuel 2:25; and Romans 9:18). God's sovereignty over everything is actually cause for hope. If He isn't, why pray to Him? God hardens hearts to set the stage for His ultimate redemptive work on the cross.
Neither unbelief nor evil deeds hinder, frustrate, or disrupt God's sovereignty. In fact, they fulfill His purposes (1 Peter 2:8).
Of the antichrist, God says, "Do not fear what you are about to suffer. Behold, the devil is about to cast some of you into prison, so that you will be tested, and you will have tribulation for ten days. Be faithful until death, and I will give you the crown of life"
Second Corinthians 4:4, 6 says, "... the god of this world has blinded the minds of the unbelievers, to keep them from seeing the light of the gospel of the glory of Christ, who is the image of God ... For God, who said, 'Let light shine out of darkness,' has shone in our hearts to give the light of the knowledge of the glory of God in the face of Jesus Christ."
Satan has blinded unbelievers so they can't see the glory of Christ, but when Jesus gives us a new heart, we believe, we respond to the Shepherd's voice.
John concludes chapter 12 stressing the importance of coming to saving faith.
Verses 42-43 say, "Nevertheless, many even of the authorities believed in him, but for fear of the Pharisees they did not confess it, so that they would not be put out of the synagogue; for they loved the glory that comes from man more than the glory that comes from God."
Though there's debate about who John is referring to, I believe he's talking about those who don't have genuine faith, only head knowledge. They know some facts about Jesus, but they would never stand for what they believe in the face of opposition. There are examples of this type of "faith" in John 2:23-25 and John 8:31-59.
What is genuine faith?
John 12:44-45 says, "And Jesus cried out and said, 'Whoever believes in me, believes not in me but in him who sent me. And whoever sees me sees him who sent me.'"
To believe in Jesus as God is to believe in the Father who sent Him, which is the only faith that results in eternal life. "This is eternal life, that they may know You, the only true God, and Jesus Christ whom You have sent"
John 12:46 says, "I [Jesus] have come into the world as light, so that whoever believes in me may not remain in darkness."
A world without Christ is in spiritual and moral darkness. We must come into the Light.
Verses 47-48 of John 12 say, "If anyone hears my words and does not keep them, I do not judge him; for I did not come to judge the world but to save the world. The one who rejects me and does not receive my words has a judge; the word that I have spoken will judge him on the last day."
John Calvin had something very interesting to say about this passage. He's often portrayed as a cold-hearted theologian who denied human choice and believed God chose the elect and condemned the rest. But listen to Calvin's comments on Jesus' appeal to His enemies here :
- "Why then does Christ not choose to condemn them? It is because he lays aside for a time the office of a judge, and offers salvation to all without reserve, and stretches out his arms to embrace all, that all may be the more encouraged to repent … No man, therefore, is condemned on account of having despised the Gospel, except he who, disdaining the lovely message of salvation, has chosen of his own accord to draw down destruction on himself." 
Calvin then paraphrases Jesus saying:
- "Burning with ardent desire to promote your salvation, I do indeed abstain from exercising my right to condemn you, and am entirely employed in saving what is lost; but do not think that you have escaped out of the hands of God; for though I should altogether hold my peace, the word alone, which you have despised, is sufficient to judge you." 
John 12:49-50 says, "For I have not spoken on my own authority, but the Father who sent me has himself given me a commandment—what to say and what to speak. And I know that his commandment is eternal life. What I say, therefore, I say as the Father has told me."
Here we see John's final passionate appeal for people to believe in Jesus. Believe in Him because He's one with the Father. Believe in Him because He's the light of the world. Believe because, if you reject Him, you'll be judged. Believe because He's the giver of eternal life.
Whose hearts are hardened?
Those who care more about the opinion of others than God's.
John 5:44 says, "How can you believe, when you receive glory from one another and do not seek the glory that comes from the only God?"
In Mark 8:38 He says, "For whoever is ashamed of me and of my words in this adulterous and sinful generation, of him will the Son of Man also be ashamed when he comes in the glory of his Father with the holy angels."
Jesus Christ is so central to history, that no one can afford to turn away from Him. No one should be ashamed of Him. No one should fear losing the glory of men or even their life. If we try to live our life without Christ by merely paying Him lip service—showing up to church at Christmas and Easter, praying only when we need something, tithing to get a tax break, helping others only when it benefits ourselves—when you take your last breath, you'll discover what you were doing is deceiving yourself.
Humbly ask God to give you spiritual eyes to see and spiritual ears to hear, to soften your heart and bring you into the Light—and He will!
 Calvin's Commentaries [Baker], pp. 50, 51
 Commentary on John - Volume 1; p. 51 - John 12:48