Universalism is the religious doctrine that all people will eventually be saved and go on to everlasting life. It contrasts with the doctrines of eternal damnation and annihilationism. Simply put, universalism is a denial of the existence of Hell, even though Jesus repeatedly emphasized its very real existence. The term universalism is normally used in the context of Christianity, but can also be applied to non-Christian religions which believe that everyone will be saved and go to heaven.
Universalism is rejected by most Christian denominations, although a small minority have taught it - it is popular at the more liberal extremes of Christian thought, such as Unitarian Universalism. It can be traced back to the first few centuries of the church, but has always been a minority view. It has been criticized as destroying the motivation for ethical and religious behaviour and individual responsibility - if it doesn't matter what I do or what I believe, and I will go to heaven anyway, then in this life I can do whatever I want and believe whatever I want and none of it matters.
Universalists either reject the idea of punishment after death entirely; or else they believe in hell, but believe it has only a limited duration, after which everyone in hell is admitted to heaven.
Compare universalism to annihilationism - annihilationists also deny eternal punishment, but they do not believe in eternal reward for the damned either. In universalism, everyone is eventually saved. In annihilationism, the saved go to heaven, the damned eternally cease to exist, either immediately after death, or at the end of a temporary period of punishment in hell. The Catholic belief in purgatory implies a limited period of punishment after death for some - but unlike universalists or annihilationists, Catholics still believe in everlasting punishment for the damned.
Origen taught that eventually everyone would repent, be saved, and be admitted into heaven - even Satan himself. But several of his views (including this) were declared heretical by many of his contemporaries in the church, even as his contributions in other areas of theology were valued.
The challenge for Christian universalism is how to explain the Biblical passages which talk about eternal damnation: Revelation 14:9-12 , Revelation 19:1-3 , Revelation 20:7-10 , Matthew 13:24-30 , Matthew 13:36-43 , Mark 9:42-48 , Luke 16:19-31 .