How do our hearts get hardened? Does rejecting the gospel over and over again harden a person’s heart?
Dead in Sins
The Bible doesn’t paint a very pretty picture of what we were like before God saved us. At one time, all of us were dead in our sins, with no hope to be saved, but God intervened and sent Jesus to give His life as a ransom for the many who would trust in Him (Mark 10:45). The Apostle Paul says that before we knew Christ, “you were dead in the trespasses and sins in which you once walked, following the course of this world, following the prince of the power of the air, the spirit that is now at work in the sons of disobedience” (Eph 2:1-2). None of us can deny that “we all once lived in the passions of our flesh, carrying out the desires of the body and the mind, and were by nature children of wrath, like the rest of mankind” (Eph 2:3). This doesn’t mean we’re better than others. We’re just better off! It wasn’t our doing, but only “because of the great love with which he loved us, even when we were dead in our trespasses, made us alive together with Christ—by grace you have been saved” (Eph 2:4-5). Before Christ, we were deader than Lazarus who was four-days-dead, but after Christ, we were quickened to eternal life by God’s Spirit (Eph 2:1-2)! 
When God choose Moses to lead His people out of Egypt, Moses was reluctant (as most prophets and leaders were), and maybe we can see why. God told Moses ahead of time that Pharaoh would “not let the people go” (Ex 4:21c). It’s the same thing with Jeremiah the Prophet. God told him ahead of time that they won’t listen to him? So what’s the point? When God calls, no one can resist, but in one of the most troubling Scriptures in the Bible, God tells Moses that they won’t listen to him because God “will harden his heart, so that he will not let the people go” (Ex 4:21). How is that fair to Pharaoh if God is the One hardening his heart? First of all, Pharaoh already had a hard heart and refused to humble himself, even despite the many miracles that brought death and destruction to Egypt. Pharaoh had been using the Israelite’s as slaves to build a nation and create wealth on the blood, sweat and tears of God’s people, so the Egyptian’s hearts were already hardened. The more God’s miraculous powers were manifested, the more Pharaoh must have been convicted that this was the One, True God, but his pride didn’t allow him to admit it or to give in to God’s demand. Over time, Pharaoh’s own stubborn heart began to harden, and did so to the point that it took the very death of his son to break his will. Since God is sovereign, God had allowed Pharaoh to continue to harden his own heart. It wasn’t God hardening Pharaoh’s heart, but Pharaoh himself. As the Apostle Paul said, God gave them over to what their lusts and pride desire (Rom 1:24). Eventually, they are given over to a depraved mind, doing what they know to be wrong (Psalm 81:12; Rom 1:28), so it wasn’t God that hardened Pharaoh’s heart; it was God allowing Pharaoh to do it to himself.
Jesus spoke some very comforting words to the broken of the world (Matt 11:28-20), but He spoke some very hard words to the proud. In other words, He spoke soft words to soft hearts, but hard words to hard hearts. Those “hard words” were intended to soften their hearts, but the Jews resisted, and eventually, they too hardened their own hearts. It wasn’t Jesus that hardened their hearts; it was their own rejection of Him and all the miracles that He did. Jesus was living proof He was the Messiah, yet they willingly closed their eyes to this truth. Rejecting the truth, over and over again, makes us resistant to the truth. And if we continue to reject the truth, eventually, we’ll never accept it, even though deep inside we know it to be true. The truth sets you free (John 14:6), or it makes you really mad. When a person is full of pride, about the last thing they’ll do is to ask for forgiveness; from God and from man. But when a person is convicted by the Spirit of God, they know they must confess their sins to God ask for His forgiveness. That’s when the heart is softened and becomes pliable, so that God can shape it and mold it. Soft clay is a lot easier to work with than hard clay. And when it’s totally hardened, the time for change has passed, and if they’re not careful, they can reach a point of no return.
God’s Word gives us a sense of urgency when dealing with faith in Christ. Nobody has any idea if they’ll even be here tomorrow, or if Jesus Christ will split the sky today in His second coming. That’s why “now is the favorable time; behold, now is the day of salvation” (2 Cor 6:2b). Sin can harden our hearts too. The author of Hebrews says, “exhort one another every day, as long as it is called “today,” that none of you may be hardened by the deceitfulness of sin” (Heb 3:13). Pride can also keep us from forgiving one another, but when we harbor grudges, we’re only hurting ourselves. Having an unforgiving spirit can harden a person’s heart too because pride closes off any possibility of confessing or admitting one is wrong.
You can actually help to prevent your heart from hardening. Humility is the key, but so is transparency, or admitting one’s faults, weaknesses, and sins, but unless God has changed, and I know He has not, you can still come to Him and ask for forgiveness and receive a cleansing (1 John 1:9). If you are not saved, right now, in fact, today, there is still time to repent and trust in Christ. As for tomorrow, who knows if it that day will even come! Believe on the Lord Jesus Christ today, and you will be saved.
Here is some related reading for you: What Does it Mean to Have a Hardened Heart? 
Resource – Scripture quotations are from The Holy Bible, English Standard Version® (ESV®), Crossway Bibles. (2007). ESV: Study Bible: English standard version. Wheaton, Ill: Crossway Bibles. Used by permission. All rights reserved.