Universalism is the teaching that all people will be saved. Some say that it is through the atonement of Jesus that all will ultimately be reconciled to God. Others just say that all will go to heaven sooner or later, whether or not they have trusted in or rejected Jesus as savior during their lifetime. This universal redemption will be realized in the future where God will bring all people to repentance. This repentance can happen while a person lives or after he has died and lived again in the millennium (as some "Christian universalists" claim) or some future state. Additionally, a few universalists even maintain that Satan and all demons will likewise be reconciled to God.
Nevertheless, both facets of universalistic belief are in serious error. People will suffer eternal damnation (Rev. 14:11) and the demonic forces have no redeemer. But, in my opinion, though universalism is a grave error, holding to the idea that all will be saved in itself does not automatically make someone a non-Christian. Please see Can a Christian be a Universalist?
However, there are those within the universalist camp (who claim to be Christian) who also deny the doctrine of the Trinity and, thereby, the incarnation of the Word of God as God the Son. They also deny the personhood and deity of the Holy Spirit. Usually, these denials are held by Unitarian Universalists, though others who are not of the Universalist camp also deny the Trinity. Those who deny these essentials cannot be classified as Christians.
To deny the deity of Christ is to deny one of the essential doctrines of salvation. In this sense, those universalists who deny the deity of Christ are in a false religious belief system. Of course, when one essential doctrine is denied, many other historic biblical doctrines are also denied and salvation is void because the object of faith is false.
There is no official "Universal Salvation Church" denomination, but there is a Unitarian Universalist Association (UUA). The UUA can be classified as non-Christian because it denies the deity of Christ, the personhood of the Holy Spirit, etc.
It is not possible to categorize all universalists into one tidy doctrinal category. Its adherents vary in belief. Some are Arian (God is one person, Jesus is a creation). Some are Trinitarian. Others even lean toward new age concepts of man's divinity.
So, universalism is not really a doctrine that identifies a group. Rather, it is a doctrine of different, even contradictory groups, who all claim universalism.
The problem with words
The cults are particularly guilty of using biblical words with non-biblical definitions. This is absolutely necessary among them in order to maintain some sort of internal consistency of theology. So too, with many universalists. Hell can mean non-existence, after-life consciousness, or this present life on earth. Some universalists believe that all punishment is accomplished here on earth, while others believe it is future event with a loss of rewards, and not a physical punishment. The punishment in both groups is corrective and limited. It will last only as long, and only be as severe, as it takes to accomplish its corrective purpose, which is to bring all mankind to a state of holiness and happiness in obedience to God. Of course, the problem with this is that it strongly suggests that a person is made worthy to be with God through his own sufferings and corrections in the afterlife.
In universalism, the word "eternal" means "without end" when it comes to salvation, but not when referring to damnation, even though the same word is used for both and in the same context (Matt. 25:46). Universalists divide history and the future into different "eons" or "ages" and assert that punishment is "age-lasting," not eternal. The term "Son of God" is claimed by all groups as an accurate description of Jesus, yet to some, it means a created being, and to others, it means God in flesh. Therefore, determining which belief is held by which universalist is often difficult and it requires digging.
Universalists often use the most negative terms to represent historic positions they disagree with. For example, regarding the damnation of the unsaved, instead of saying that historic Christianity teaches that those who reject Christ will suffer eternal damnation, they frequently say that historic Christianity teaches that "God can't save everyone and wants to torture most of humanity forever." Or, it is often implied that God will not torture people forever because "God is not sadistic enough to send people to hell." Such emotionally slanted words reveal a hostile bias against historic doctrines and is an unfair description of those beliefs. It is a surprisingly common tactic among universalists which demonstrates their lack of objectivity and sheds an automatic cloud of doubt upon their observations.
As you can see, universalism covers a wide range of beliefs. Though belief in universalism, in and of itself, does not automatically void salvation, it has the potential danger of allowing false teachers to abide alongside true believers as well as deny basic Christian teaching such as eternal damnation. Therefore, to determine if a universalist is Christian, you must delve further into other areas of his belief system.