I am a diocesan priest in the parish of Cyrill and Method in the Archdiocese of Vienna, and a teacher at the In the posts Is predictability incompatible with responsibility for sin and The Difference Between Truth and Error, I argued that external causes (genetics, upbringing, circumstances, etc.) that are not the result of a person's will, and yet make it more likely that that person will commit an objectively evil act, decrease the voluntariness of that act. As pointed out in a comment, this principle could be used to absolve almost everyone of responsibility for sin, and in fact is not infrequently so used in modern times. Clarence Darrow argued in the trial of Richard Loeb and Nathan Leopold for murder: "Nature is strong and she is pitiless. She works in mysterious ways, and we are her victims. We have not much to do with it ourselves. Nature takes this job in hand, and we only play our parts…. What had this boy had to do with it? He was not his own father; he was not his own mother….All of this was handed to him. He did not surround himself with governesses and wealth. He did not make himself. And yet he is to be compelled to pay." Though this argument was not credited by the judge, who stated that it did not pertain to the court to make a judgment of ultimate responsibility, one might argue that God judges precisely on the basis of ultimate responsibility, and does not condemn a person for an act to which he was largely led by upbringing, circumstances, etc.
What to say about this? For the sake of illustration, and in the spirit of modern science, let's describe a person's amount of responsibility with a number from 0 to 1, where 0 means the person is totally free of responsibility, and 1 means he is totally responsible. Where is the dividing line between the acts of sin for which God holds a man accountable, and the acts for which God does not hold a man accountable? If God does not condemn a person for sin unless he is at least 0.99999 responsible for it, the view that nearly everyone goes to heaven might be quite a bit more plausible. Conversely, if God condemns a person for sin if he is even 0.00001 responsible for it, the view that nearly everyone goes to hell would become more likely.
We do not have direct insight into the mind and judgment of God on this matter, but we have hints about his answer; God's answer to the question of this post would seem to be similar to his answer to the question "Are they few who will be saved?"
Strive to enter by the narrow door; for many, I tell you, will seek to enter and will not be able. (Luke 13:24).
Whatever God's future judgment may be about those who are led to sin through their upbringing, their culture, and so on, our earnest concern should be to avoid sin and to follow Christ, and similarly to draw others away from sin and to Christ.
Supposing that due overwhelmingly to external causes (upbringing, culture, bad literature, etc.) a man turned his heart against God. That is, suppose that in his inmost will he chose to be away from God and with Satan and all that is evil.
In this case, there is not just a question of God's holding him accountable, as though his problem were a debt of which he might be acquitted. The question has to do with his heart: he is in fact pointed away from God, however he got there.
Admitting him to heaven at his death would therefore involve not only acquitting him but also interiorly, miraculously, and suddenly changing his heart to point towards God.
Does this change the analysis at all, or is this a red herring?
I did grant that the argument does not manifestly prove that it is impossible, through a studied process making careful use of the various faults a person commits, to almost inevitably lead him into sin in such a way that he remains substantially responsible for that sin. But let's suppose this is at any rate not so–the explanation of the sin is overwhelmingly external–and that the sin in question is hatred of God, devotion to Satan.
One one of responding to the scenario is to say that it is impossible for someone "in his inmost will" to choose to be away from God due overwhelmingly to external causes.
Another way (really another aspect) is to say something similar to what Fulton Sheen suggested about the church: "There are not over a hundred people in the United States who hate the Catholic Church. There are millions, however, who hate what they wrongly believe to be the Catholic Church." (Preface to Radio Replies).
If hatred for God and devotion to Satan are inevitably or nearly inevitably produced in a man by others, this can only be by means of a certain trickery. He would not truly hate God for what he is or in a manner at odds with God's goodness, but hate what he believed God to be; and similarly, would not be devoted to the devil in a manner at odds with the devil's badness, but be devoted to what he believes the devil is.
Consequently, standing before Christ and knowing him for who he is, and the world as it is, the hatred would immediately vanish. It is something like the case of a man who long nourished anger and hatred for another man because he thinks that man deliberately and gravely cheated him, who finds out that it was all a plot by a third man to destroy his relationship. At least at the highest level of his will, the anger and hatred would immediately be re-directed towards the true evil. Possibly at a lower level he would still be upset at the man he previously thought had cheated him, feeling subconsciously that he should have been able to do something about it.
If such disordered feelings remained in his soul at a lower level.
It may be that he would go through a kind of purgatory of his own will, not so much for punishment as for purification. But it is hard to say, since we cannot know much about the inner workings of the soul when separate from the body.