Satan is a blessing in disguise, he helps bring out the goodness in people by trying to bring out the badness. In book one Milton's writes, "What reinforcement we may gain from hope, if not what resolution from despair." Which basically reads if humans find a way to control their demons or anguish, they restore hope in themselves to remain good. God's ideal thinking for letting Satan and his fallen angles free is so that his evil deeds could be twisted good. In other words, Satan helps bring consequence to one's actions. Of course, sadly not everyone could resist Satan's aims so there are mortals in both Heaven and Hell. As Milton put it, "the mind is its own place, and in itself can make a Heaven of Hell or Hell of Heaven." So with Satan in Hell and God in Heaven humans are challenged by both, where the mortals decides to stay in the afterlife is up to them.
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It was an issue that was not spoken about greatly in that time due to the focus on over seas conflicts.
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