Because there are locations in the world where a person can live their life and not be exposed to the Gospel, and people lived before Christ came. What happens (or happened) to those souls when they die (or died).
IOW, if a person does not get a shot at knowing Christ, what happens to their soul when they die? I hear this argument a lot. You seem to be the guy to ask.
Oh, I understand the argument and have had that conversation with both of the posters you mentioned, I do believe.
You just referred to the argument as "valid". I take issue with that claim, so ask why you would call it "valid".
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The claim is generally that people who never hear the gospel will go to hell and that's not fair.
Scripture isn't clear on the matter, so I can't speak in definitives. It may be that a person who never heard the Biblical account may respond to God progressively enough to reach salvation through Jesus while never hearing of Jesus as Jesus. (This is my personal hope.)
Everyone who was saved in the OT was saved by faith in Jesus, not named yet as Jesus, but Jesus nonetheless. The book of Hebrews is explicit that every OT salvation occurred via faith in Jesus as He had revealed Himself to them at the time.
It's also conceivable that every person who dies without hearing the gospel goes directly to Hell, do not pass go, do not collect $200. Paul, in Romans, makes it clear that those who were given the law are judged by the law and those who were not given the law are judged by their own conscience. Some are condemned by their knowledge of scripture and some by their knowledge of their own moral failures.
The problem for those using this example against God is... What makes it "valid", as you said?
They never get around to making it a valid argument. They just play on (fallen) human preferences and hope that the conversation stays at the level of what we (fallen) humans think to be acceptable--let's say "preferable".
(1) God created the universe for the purpose He has for the Universe, so He had the right to create it as He saw fit.
(2) The accusers in the room are very liberal in telling us what's "right" and "wrong" for God to do. They're the first to stand up and tell us when God is acting unethical and immoral.... But this is only after informing the world that morality is relative and there is no objective standard to it.
So, all they are actually saying when they accuse God is that He didn't do things according to their preference. Well, duh.
(3) We will take PKT's example since it's a recent example. PKT just said (yesterday?) in this very thread that the great stumbling block for most people per Christian theology is that a child rapist could repent and be forgiven, yet Gandhi would be in Hell right now.
That's an appeal to justice. PKT and Septic would be standing on their view of justice there. They want justice, not grace.
Scripture tells us that every person who ever goes to Hell is judged based on God's justice, and it is just for them to be there. Further, scripture tells us that every person who is saved is saved by grace.
So, the argument seems to be switched now, instead of wanting justice, they all want everyone to get grace. You see? Grace is a stumbling block when it's convenient to stumble on it. And justice is a stumbling block when it's convenient to stumble on it.
So, the unpopular answer, yet the answer I believe to be true? I don't understand exactly how it all works. As part of my faith, I trust God's judgment. Anyone who goes to hell is there because of what they've done. Anyone who is saved is saved despite themselves, based solely on God's grace. Theologically, it's a bit ridiculous to say that God is forced to issue His grace to whom, and in ways that, He chooses not to.
The accuser's belief in a relative morality makes the entire conversation a non-starter if they choose to be honest about their beliefs and actually live those beliefs out to their logical conclusions. (If morality is relative and mere preferences, then they have no platform to say their "preference" is more valid than God's "preference".)