There is a seemingly potent objection to universal salvation of humans, namely that if demons, whose angelic nature is at least somewhat superior to the human nature, go to hell, then why not humans?
In fact, there are two different hells: the demonic hell, DH, and the human hell, HH, and they differ considerably. For example, some Christians say that hell-bound humans will suffer “eternal separation from God” and leave it at that. That is only partly true. For separation from God is what is called “pain of loss,” or the consciousness of failing to enjoy the pleasures of heaven. It does not entail any “pain of sense” or the torture by hellfire. DH is characterized by the former only; HH, by both.
For humans, pain of sense without pain of loss is purgatory.
This is because the natures of demons, good angels, and humans differ. For a demon who voluntarily chose to descend morally, DH is a natural end state. Lucifer knew perfectly well what fate he was sealing for himself. As a result, his nature would be preserved, and there cannot be any pain for a being in its natural state. But hell for a human is a different state entirely: since humans naturally ascend (such as from a single-celled organism to an inhabitant of heaven), should they reject this natural impulse and do evil instead, their nature will be destroyed, and they will endure the inconceivable pain of a corrupted soul in HH.
No creature whatsoever would willingly choose anything like the human hell. It defies reason. Hence, it is still possible to assert that no human will be damned.