J. I. Packer:
A universalist is someone who believes that every human being whom God has created or will create will finally come to enjoy the everlasting salvation into which Christians enter here and now. Universalism is the recognized name for this belief. . . .
Among Christian theological options it appears as an extreme optimism of grace, or perhaps of nature, and sometimes, it seems, or both. But in itself it is a revisionist challenge to orthodoxy, whether Roman Catholic, Eastern Orthodox, or Protestant evangelical; for the church has officially rated universalism a heresy ever since the second Council of Constantinople (the fifth ecumenical council, A.D. 553), when the doctrine of apokatastasis (the universal return to God and restoration of all souls) that Origen taught was anathematized.
—J. I. Packer, “Universalism: Will Everyone Ultimately Be Saved? in Hell Under Fire, ed. Morgan and Peterson (Zondervan, 2004), p. 170.
For audio of Packer teaching about hell—including a refutation of the idea that the Greek words aion and aionos don’t really mean “eternal”—take a look at this helpful post by Martin Downes.