Someone asked me on the CHNI board whether a person could theoretically go to hell because we didn't pray for them, or pray enough, etc., so that they did not repent; and noted that this would seem unfair for God to "allow." I replied:
You're assuming several things here that do not follow.
First, it's not a matter of God "allowing" someone to go to hell (as if He were to blame for it). If they go there it is because they wanted to live without God. Like C.S. Lewis once wrote with great insight, "the doors of hell are locked on the inside."
Second, all grace, including that conveyed through prayer, comes from God. Since God knows everything (including even possibilities and contingencies and all possible outcomes), then if He foreknew that Person A would not pray for someone else, He could easily "arrange" things in His Providence so that Person B would do so.
Third, in the final analysis, whether we are saved will always depend on God's grace, but it is our decision whether to accept this grace or not. The grace will be provided. It doesn't ultimately depend on another person, so that if they fail, we go to hell. It just doesn't work that way. That would be contrary to God's mercy, to let eternal salvation rest on the week reed of third parties.
The proper way to express our choice with regard to eternal destiny would be to say (using myself): "Dave did not accept the grace and the free option of going to heaven that God provided for him, and so went to hell." We all stand before God alone, in the end, as many old folk songs point out. "You got to walk that lonesome valley by yourself," etc.
We mustn't confine ourselves to a merely a human way of looking at things. God has a completely different perspective (as revealed in Holy Scripture).
Our task is to pray. We know that for sure. It's not in our hands who is saved and damned. We can help provide the avenues of grace and blessing that God has for each individual. Their own final destiny rests with them and with God. We are to pray, do good works, love, do penance on behalf of others, listen, assist, and share the Good News at every reasonable opportunity. Then we know that we are fulfilling our responsibility and holding up "our end of the bargain," so to speak. How I look at it is that these good things make the (Christian) path easier for others to follow, but do not determine their path; let alone whether they go to heaven or hell.
Part of the message of Mary's appearance at Fatima was brought up: "Souls dropping into hell like snowflakes, because no one will pray for them" (Brother Carlo). It was suggested that Mary's message may have been a motivational tool of sorts.
I like this way of expressing it. It isn't that what Our Lady was saying there is "not true"; rather, one has to interpret it correctly. Remember, a common teaching method among the Jews was the strong compare and contrast motif, or the hyperbole. "If your eye offends you, pluck it out." "If you have enough faith, you can remove this mountain," etc.
Books like Proverbs and Psalms present a stark contrast between the "evil, wicked fools" and the "righteous" and the "wise." That's proverbial language, though, which intends to convey generalities. We all know that human beings are quite the mixture of good and evil and often are more like "shades of grey" rather than pure and righteous and holy vs. utterly evil and wicked. Alexander Solzhenitsyn, the famous Orthodox writer and dissident against Communism stated that "the line between good and evil runs through every human heart."
It is the extreme contrast that is the motivational tool to reform behavior and to express the urgency and importance of action on behalf of others. In this matter, there is indeed a relationship between our prayers and graces always caused and offered by God through us to others. The same applies to penitential works, and even suffering (redemptive suffering). God designed things that way, so that we can all be involved in the marvelous process of His graces. He wanted salvation and redemption to be a community or organic effort, not a bunch of atomistic individuals. We help each other. We are "our brother's keepers."
I think we can, therefore, quite possibly interpret this aspect of the message of Fatima (perhaps in part, at the least) as "if no one prays for a soul, that soul will go to hell." But that someone need not be restricted to individual persons. It is a collective notion: "every soul needs the assistance of prayer [general]; therefore you [particular; part of the collective] should pray." It doesn't logically follow, however, that if we as individuals fail to pray for an individual person, that that person will go to hell, because, as I noted before, God in His Providence will simply cause or urge another person to do so.
And even then, a person could receive all kinds of graces through prayer and what not, yet still reject God and salvation, because God gave us the free will to do so, so that following Him would be a meaningful choice, not a question of God's pressing a button and our not being able to refuse, as if we were robots. After all, look at Satan himself: he was in heaven with God and had everything he could possibly need or want. But he wanted to be in God's place, so it wasn't enough for him. Causation in such matters of final salvation is not strictly a matter of us (i.e., we as individuals) and them.
Or, on a more human level: we have all seen families where two children were raised the same way, yet one rebels and gets into heavy sin, while the other stays faithful to the Church and Christian moral teachings. Free will . . . both were given the same "graces" so to speak, by the parents, but the outcome was different because they decided in the end which way they would go.
The final destiny of the person rests on his or her acceptance or rejection of God and His grace. We know that God predestines those who will be saved, to salvation, yet (paradoxically) not without their choice and free will assent. How this works out in fine detail is a matter of debate in Catholic theology, between Thomists and Molinists (and I am, by the way, of the latter camp). But (in Catholic teaching, in contrast to Calvinist) He predestines no one to hell.