Naturally, enemies of Universal Salvation want to make it sound like a Pagan belief system. But this is a manipulation of the facts. I can say firmly that few if any Ancient Pagans believed in what I mean by Universal Salvation.
In Egyptian mythology the souls of the sinful were devoured by Ammit.
Greek mythology taught the opposite of Unviersalism, it taught that there was no hope of escape from Hades for anyone. That is the ultimate moral of the story of Orpheus. And my refutation of those Christ-Mythers who say the Christian doctrine of the Harrowing of Hell is just copying Orpheus, is to say that Orpheus failed to free his Bride from Hades, Jesus succeeded.
Now you can call that Universalist by looking at it as everyone has the same fate basically. But The Gospel as I view it was originally a deliberate rejection of the philosophy that death is a natural part of life we just need to accept, which was also the moral of the Epic of Gilgamesh, or Izanami's fate in Japanese mythology. The Gospel was the promise that ALL will be freed from Sheol. Paul declared in 1 Corinthians 15 that Death had no Sting and Hades has no Victory.
And this is part of why I view Tolkien and Lewis as very Paganized Christians, both have death existing even among races supposedly not effected by The Fall. In Tolkien's Arda mythology Death is Man's Gift not a punishment, which the Eldar seem to envy even though they can die, they just aren't guaranteed to. And with Lewis, Out of The Silent Planet is supposed to present his view of what an unfallen world would be like, and depicts Death as a natural part of creation.
And people also want to link Universalism to Plato, even though the most openly Platonic Church Father, Augustine, was also the harshest enemy of Universalism.
Platonic philosophy is tied to Gnosticism and any other belief that there is no Physical Bodily Resurrection, that the Resurrection merely refers to the liberation of our Spirit/Soul from the material world. In counterpart to that Webster Tarpley, a Catholic and Plato fanboy, accuses Gerrard Winstanley of believing in "Dead Souls", what he leaves out is Gerrard Winstanley was a Universalist, he believed all will be Resurrected to Eternal Life. I don't think one's view on how conscious the Soul is between death and resurrection is that important, it's one's view on the Resurrection that is vitally important. Platonic philosophy allows no bodily resurrection, and Augustine as the first Amillenial laid the groundwork for a Christian version of that. Plato (like Origen) may have been nicer then Augustine in not condemning some Souls to eternal torment, but he didn't allow any a true Resurrection.
The understanding of the After Life held by casual Christianity, which forgets that our ultimate goal is the restoration of this world, that simply sees it as the good go to heaven and the bad to hell. Is actually Zoroastrian in origin.
Islam is totally incompatible with true Unviersalism. The closest they can come is believing that Jews and Christians, the People of the Book, can achieve Salvation without becoming Muslims. But they allow no such hope for people outside of the Abrahamic tradition.
The ability of modern Pagans (and I define modern here as starting with the Renaissance), including the modern understanding of Shintoism that we often see reflected in Anime, to be willing to be Universalist is something I view as possible only because of Christianity's influence on the world. But even then the idea that Death is a natural intended part of how the world works often remains.