I think a lot of Christians try to act like modern jews, in that they want to box God into some legal checklist of "You go to hell if...But you go to heaven if..." In reality, Jesus loves all of humanity and conquered death itself, and he can save or condemn anyone he chooses, and it's up to him how he decides to handle people who never heard of him (regardless of what point in history they happened to live).
There's no higher law. He's the judge, jury, and executioner. He's the creator, and he's also the destroyer. He can save someone who never heard of him, and he can condemn someone who claimed to follow him, and what is anyone else going to do about it? There's no appeals court, no resistance movement, no hiding, and no escape. If you want to avoid hell, he's the only ticket.
That said, the bar for salvation (i.e. avoiding hell) is pretty low. He already did that work for us. But of course, those who really believe that's true tend to live differently, with humble righteousness as a goal every moment of their lives.
So to your original point, "What about the Spartans who were good people and tried to do the right thing?" Well, I wouldn't be surprised to see them in heaven. But ultimately that situation gets decided by Jesus, not me or any other mere human.