Eternal hell in the Qur'án?

Unbelief is hell. As long as you cannot or will not recognize the Beloved, you are forever deprived of what you long for most.

Imagine you’re in a desert, dying of thirst. There is a pool of crystal, pure water in front of you, but you’re convinced it’s poison and want nothing to do with it. As long as you persist in your refusal, you remain in torment; the moment you drink, it ends.

Said another way: The soul of an unbeliever can never reach heaven, because the very essence of heaven is recognition of its Lord. This does mean the soul cannot someday, by the grace of God, believe; only that it can never advance in a state of unbelief.