(Pro 29:1 KJV) He, that being often reproved hardeneth his neck, shall suddenly be destroyed, and that without remedy.
This verse is a two edged sword. The first edge teaches the patience of God. God is giving people multiplied chances to repent. We are not unable to listen to the things of God. Even if they will not admit it, people are aware of right and wrong. The unbeliever “hardeneth” (7185) his heart deliberately.
Pharaoh hardened his heart and then God continued to harden it. God seems to respond to the desire of our hearts in ways that we don’t expect.
Hebrews 6:4 speaks of the impossibility of renewing your faith. The key here is the idea of being "reproved" (8433b), not just a casual rejection of correction. The NASB adds the word “much”. “Reproof” (8433b) seems to have the idea of argument. I get the idea that this refers to a person who has been reasoned with for a period of time and refuses to listen or change.
Judgment will come “suddenly” (6621). The hardened man will be “broken” (7665).
The frightening part is when we are broken “without" (beyond NASB) (369) “remedy” (4832). This could be taken in the same way as Jesus’ answer to the question of how anyone can be saved if rich men are condemned. Several places the Bible says that God does not listen to the prayers of unbelievers. It is express different ways and in both Old and New Testaments. The warning is again that God does not play games. He is patient. He is merciful. But there will come a point where it will be too late to call upon that patience and mercy.