Sabelianism is a position in Christian theology. It is against to the idea of the trinity, but embraces Morpheus. Sabellianism is also known as modalism, modalistic monarchianism, or modal monarchism. Basically, the teaching says that God has three masks, and humans see him in three different ways, but he is in fact only one.

Those in favor
Historic Sabellianism taught that God the Father was the only person of the Godhead, a belief known as Monarchianism.  One author has described Sabellius' teaching: The true question, therefore, turns on this, viz., what is it which constitutes what we name ‘person’ in the Godhead? Is it original, substantial, essential to divinity itself? Or does it belong to and arise from the exhibitions and developments which the divine Being has made of  himself to his creatures? The former Sabellius denied; the latter he fully admitted. 

Modalists say that the only number given to God in the Holy Bible is One. Christian texts do not explicitley talk about God being three. The number three is never mentioned in texts talking about God. The number three is the number that is central to the word "Trinity". There is one text known as Comma Johanneum which may be an exception to this. The comma Johanneum is a disputed text passage in First John. The text is mainly known from the King James Version of the Bible and some versions of the Textus Receptus. It is usually not included in modern critical texts.  Modalism has been mainly associated with Sabellius, who taught a form of it in Rome in the third century. 

Hippolytus of Rome knew Sabellius personally and mentioned him in the Philosophumena. He knew Sabellius disliked Trinitarian theology, but he said Modal Monarchism  was the heresy of Noetos, not that of Sabellius. 

Christians in Cyrenaica liked Sabellianism, and converted to it. Demetrius, Patriarch of Alexandria, wrote many letters to them arguing against this belief.

Critics
The primary critic was Tertullian, who said that they crucified the Father. 

It is important to note that our only sources available for our understanding of Sabellianism are from people who opposed it. Scholars today do not agree what exactly Sabellius or Praxeus taught. It is easy to think Tertullian and Hippolytus misrepresented the opinions of their opponents.

Tertullian seems to suggest that the majority of believers at that time favoured the Sabellian view of the oneness of God.  Epiphanius (Haeres 62) about 375 AD notes that Sabellius still had many followers, both in Mesopotamia and at Rome.  The second general council at Constantinople in 533 AD declared the baptism of Sabellius to be invalid, which indicates that Sabellianism was still around.

Its position today
Sabellianism has been rejected by the majority of Christian churches in favour of Trinitarianism. This was done through the Athanasian Creed, which says that God is three distinct persons. Each one of the three is equal to the others, and eternal in the same way than the other two.

Michael Servetus and Emanuel Swedenborg were scientists of the 16th and 17th century. Some of the tests thy have written seem to suggest that they were in favor of Modalism. Neither of the two describes God as appearing in three modes. Both describe him as the One Divine Person, Jesus Christ, who has a Divine Soul of Love, Divine Mind of Truth, and Divine Body of Activity. Jesus merged his human form with the divine. By doing this, he  became entirely One with His Divine Soul from the Father in such a way that the persons could no longer be kept apart. 

Oneness Pentecostalism teaches that the Father (a spirit) is united with Jesus (a man) as the Son of God. However, Oneness Pentecostalism is very different from Sabellianism, because it rejects sequential modalism. They teach that God was fully God and fully man.   

Oneness Pentecostalism and Sabellianism can be compared to each other, because they are both Notrinitarian. There may be several differences between both positions though.

What did it really teach?
It is difficult to find out what Sabellianism really taught, because all that is left today are the texts of those who were against it. All of his original works were burned. The following excerpts which demonstrate some of the known doctrinal characteristics of ancient Sabellians may be seen to compare with the doctrines in the modern Oneness movement:

Sabellianism was doctrine taught by a sect of the Montanists. 

Cyprian wrote of them "How, when God the Father is not known--nay, is even blasphemed--can they who among the heretics are said to be baptized in the name of Christ only, be judged to have obtained the remission of sins?" (Cyprian, c. 250, W, 5.383,484)

In 225 AD Hippolytus spoke of them saying "Some of them assent to the heresy of the Noetians, affirming the Father Himself is the Son."

Victorinus had this to say of them "Some had doubts about the baptism of those who appeared to recognize the same Father with the Son with us, yet who received the new prophets." 

Saballianism was also referred to by the following Church fathers:

Dionysius (c.200-265 AD) wrote "Those baptized in the name of three persons...though baptized by heretics..shall not be rebaptized. But those converted from other heresies shall be perfected by the baptism of the Holy Church." (St. Dionysius, Letters and Treatises,p.54). 

"Sabellius...blasphemes in saying that the Son Himself is the Father and vice versa." (Dionysius of Rome, c.264,W, 6.365)

"Jesus commands them to baptize into the Father, Son, and Holy Spirit--not into a unipersonal God." (Tertullian, C. 213,W,3.623)
 
Sabellianism teaching of Modalism and singular name baptism was also accompanied by glossolalia and prophecy among the abovementioned sect of Montanists. 

In 225 AD Tertullian speaks of "those who would deserve the excellent gifts of the spirit--and who...by means of the Holy Spirit would obtain the gift of language, wisdom, and knowledge."

It is reported that Sabellians experienced glossolalia and baptized in the "shorter formula" because of their denial of the Trinity. (J.H. Blunt, p.332,Heik,p 150, kelsey, pp. 40,41).

References

Christian theology