Why do people reject Jesus when the evidence that He is Lord and Savior is right in front of them?
Consider Moses's words in Deuteronomy 29:2-4: "And Moses summoned all Israel and said to them: 'You have seen all that the Lord did before your eyes in the land of Egypt, to Pharaoh and to all his servants and to all his land, the great trials that your eyes saw, the signs, and those great wonders. But to this day the Lord has not given you a heart to understand or eyes to see or ears to hear.'"
Compare these to John's words in John 12:40: "He has blinded their eyes and hardened their heart, lest they see with their eyes, and understand with their heart, and turn, and I would heal them."
Many people have misunderstood this verse because it sounds like it's God's fault that the people don't believe. So, instead of accepting it for what it says, they say it's talking about something else. However, the phraseology, language, and context all point to God preventing the people from believing.
So, did God harden their hearts? Did God make them blind and deaf? Yes and no. We must remember, from the moment we're born, every one of us is spiritually blind and deaf, dead in our trespasses and sins. We're completely unable to see or hear Christ. We're unable to believe in Him. The only way any of us can see Jesus for who He really is, is if the Holy Spirit removes the veil from our eyes, unplugs our ears, and changes our heart of stone into a heart of flesh.
So, on one hand, it's our fault, as fallen depraved sinners, for being spiritually blind and deaf. On the other, God allowed us to remain that way.
How did God harden their hearts? How did He blind them? He allowed them to continue in their sins. Sin causes hearts to grow hard. Still, "if we confess our sins, [Jesus] is faithful and just and will forgive us our sins" (1 John 1:9). However, if we don't confess our sins, they have a cumulative and desensitizing effect.
Scripture makes it clear that, if we relentlessly continue to engage in sin, there will come a time when God will give us over to our "debased mind" and let us have it our way. That's what happened with Israel. They continued to engage in sin to the point that God let them surrender to it. The effects of this can be seen in our passage. The people see their Messiah, the Son of God, God Himself, standing before them, and, yet, they reject Him.
And you know what's interesting? Jesus cites this very same passage in Isaiah 6 as His reason for speaking to the multitudes in parables. That way, only those who had ears could hear. Only those from whom God removed the veil and unplugged the ears could understand. Paul quotes the same verse as well (Acts 28:25-27). When unbelieving Jews approached Paul, who visited him in Rome, he used this verse to justify why he turned to the Gentiles.
The point that John is making is that it doesn't matter what miracle our Lord performs. There are 125 miracles listed in the Bible from the creation of the world to cities being destroyed, from families being delivered from destruction to plagues on Egypt, from the Exodus miracles to the miracles surrounding God's prophets, from the miracles of Jesus to those of His apostles.
It doesn't matter how great and mighty a miracle is; people will not believe in Christ as their Messiah. Jesus did not meet their expectations. That was the problem with these Jews. Jesus didn't meet their expectations of a mighty, conquering political messiah.
Instead, Isaiah 53:2-3 describes him perfectly. "For he grew up before him like a young plant, and like a root out of dry ground; he had no form or majesty that we should look at him, and no beauty that we should desire him. He was despised and rejected by men, a man of sorrows and acquainted with grief; and as one from whom men hide their faces he was despised, and we esteemed him not."
Jesus did not meet their expectations for their Messiah. So, they rejected Him even though all the evidence pointed to Him.
As believers, at some point in our lives, Jesus didn't fit our expectations. At some point, we've probably gone to Jesus and believed He would fix all our problems, but everything got worse. You expected Him to save your spouse, to save your marriage, to save your kids, but, instead, your spouse left you, your children rejected the gospel and harden themselves towards you.
Can Jesus fix our problems? Yes, absolutely He can! If that's what's best for us. Having a relationship with Jesus Christ doesn't mean all our problems go away. We need to be very careful as believers not to go to Jesus with false expectations because, when He doesn't meet those expectations, it could be catastrophic to your faith.