The theology around hardening is not simple, but whatever it means it does not contradict the clear testimony of Scripture that unconverted people are dead in trespasses and sins- not sick or wounded but dead- and we are in that state from birth which is why we need to be born again.
Perhaps this analogy will be helpful. A man hears that his son, in the company of some other criminals, is on his way to commit a serious crime. He intercepts the criminals and forces his son to come home with him. He solemnly warns the others that they will suffer the consequences of their crime and appeals to them to give up their nefarious plans. However, when they refuse to listen to him, he takes his son home, 'phones the police and leaves the others to the consequences of their actions.
I say that this man has done all that could be expected of him on behalf of the other criminals. It is only their stubborn and rebellious hearts that have kept them from heeding his advice and giving up their scheme. They have no one but themselves to blame when they end up in jail.
So it is with God. God keeps no one out of heaven (John 6:37b) and indeed He sends preachers and missionaries to plead with men to repent, but men in their wickedness will not turn to Christ to be saved (John 3:19; 5:40) unless God changes their hearts (John 6:44) and this, in His inscrutable wisdom (Rom 11:33ff), He has not done for everyone, but only for those upon whom He has set His love and given to Christ to redeem (Matt 11:27; John 6:37a etc.).
So when men continually reject Christ, God's longsuffering comes to an end (Gen 6:3; Isaiah 65:2ff) and He gives them over to their wickedness (Rom 1:24 etc.) and no longer pleads with them through His word. This is what is meant by hardening.
I forsee lots of objections to this explanation, but it does seem to be the teaching of Scripture. Anyone wishing to argue should be prepared to deal with Rom 9:18ff.
:godisgood:
Steve