In Christianity, Sabellianism, (also known as modalism, modalistic monarchianism, or modal monarchism) is the nontrinitarian belief that the Heavenly Father, Resurrected Son and Holy Spirit are different modes or aspects of one God, as perceived by the believer, rather than three distinct persons in God Himself.
The term Sabellianism comes from Sabellius, a theologian and priest from the third century.
Meaning and origins
God was said to have three "faces" or "masks" (Greek πρόσωπα prosopa; Latin personae). Modalists note that the only number ascribed to God in the Holy Bible is One and that there is no inherent threeness ascribed to God explicitly in scripture.  The number three is never mentioned in relation to God in scripture, which of course is the number that is central to the word "Trinity". The only possible exceptions to this are the Great Commission Matthew 28:16-20, 2 Corinthians 13:14, and the Comma Johanneum, a disputed text passage in First John (1 John 5:7) known primarily from the King James Version and some versions of the Textus Receptus but not included in modern critical texts. Trinitarians believe that all three members of the Trinity were present as seemingly distinct beings at Jesus' baptism, and believe there is other scriptural evidence for Trinitarianism (see main page for details). Modalism has been mainly associated with Sabellius, who taught a form of it in Rome in the third century. This had come to him via the teachings of Noetus and Praxeas.
Hippolytus of Rome knew Sabellius personally and mentioned him in the Philosophumena. He knew Sabellius disliked Trinitarian theology, yet he called Modal Monarchism the heresy of Noetus, not that of Sabellius. Sabellianism was embraced by Christians in Cyrenaica, to whom Demetrius, Patriarch of Alexandria, wrote letters arguing against this belief.
The chief critic of Sabellianism was Tertullian, who labeled the movement "Patripassianism", from the Latin words pater for "father", and passus from the verb "to suffer" because it implied that the Father suffered on the Cross. It was coined by Tertullian in his work Adversus Praxeas, Chapter I, "By this Praxeas did a twofold service for the devil at Rome: he drove away prophecy, and he brought in heresy; he put to flight the Paraclete, and he crucified the Father."
It is important to note that our only sources extant for our understanding of Sabellianism are from their detractors. Scholars today are not in agreement as to what exactly Sabellius or Praxeus taught. It is easy to suppose 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 notes that the adherents of Sabellius were still to be found in great numbers, both in Mesopotamia and at Rome. The first general council at Constantinople in 381 in canon VII and the third general council at Constantinople in 680 in canon XCV declared the baptism of Sabellius to be invalid, which indicates that Sabellianism was still extant.
Historic Sabellianism taught that God the Father was the only true existence of the Godhead, a belief known as Monarchianism. One author has described Sabellius' teaching thus: 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. 
It has been noted that Greek term "homoousian" or "con-substantial", which Athanasius of Alexandria favored, was actually a term reported to be put forth by Sabellius, and was a term that many followers of Athanasius were uneasy about. Their objection to the term "homoousian" was that it was considered to be un-Scriptural, suspicious, and "of a Sabellian tendency." This was because Sabellius also considered the Father and the Son to be "one substance." Meaning that, to Sabellius, the Father and Son were one essential person.
Sabellianism has been rejected by the majority of Christian churches in favour of Trinitarianism, which was eventually defined as three distinct, co-equal, co-eternal persons by the Athanasian Creed late in the fourth century.
Both Michael Servetus and Emanuel Swedenborg have been interpreted as being proponents of Modalism. Neither, however, described God as appearing in three modes. It is not necessary to describe God in three modes to be Oneness. Both describe God as the One Divine Person, Jesus Christ, who has a Divine Soul of Love, Divine Mind of Truth, and Divine Body of Activity. Jesus, through a process of uniting his human form to the Divine, became entirely One with His Divine Soul from the Father to the point of having no distinction of personality.
Oneness Pentecostalism teaches that the Father (a spirit) is united with Jesus (a man) as the Son of God. However, Oneness Pentecostalism differs significantly by rejecting sequential modalism and by the full acceptance of the begotten humanity of the Son, not eternally begotten, who was the man Jesus and was born, crucified, and risen, and not the deity. This directly opposes Patripassianism and the pre-existence of the Son, which Sabellianism does not.
Oneness Pentecostals believe that Jesus was "Son" only when he became flesh on earth, but was the Father prior to his being made human. They refer to the Father as the "Spirit" and the Son as the "Flesh". But they believe that Jesus and the Father are one essential Person. Though operating as different "modes". Oneness Pentecostals reject the Trinity doctrine as pagan and un-Scriptural, and hold to the Jesus' Name doctrine with respect to baptisms. They are often referred to as "Modalists" or "Sabellians" or "Jesus Only". Oneness Pentecostalism can be compared to Sabellianism, or can be described as holding to a form of Sabellianism, as both are Nontrinitarian, and as both believe that Jesus was "Almighty God in the Flesh", but they do not totally identify each other.
However, it cannot be certain whether Sabellius taught a dispensational Modalism or taught what is known today as Oneness since all we have of his teaching comes through the writing of his enemies. 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 adhered to 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 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."
- Sabellianism was also referred to by the following Church fathers:
- Dionysius (c. 200-265) 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 Tertullian spoke 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).
- Greek Gospel of the Egyptians
- pgs 51-55Vladimir Lossky The Mystical Theology of the Eastern Church, SVS Press, 1997. (ISBN 0-913836-31-1) James Clarke & Co Ltd, 1991. (ISBN 0-227-67919-9)
- Moss, C. B., The Christian Faith: An Introduction to Dogmatic Theology, The Chaucer Press, London, 1943
- See, for example, Metzger, Bruce M., A Textual Commentary on the Greek New Testament [TCGNT] (2nd Edition), Stuttgart: Deutsche Bibelgesellschaft, 1994, pages 647-649.
- A History of Christianity: Volume I: Beginnings to 1500: Revised Edition pg 144-146 By Kenneth S. Latourette Published by HarperCollins, 1975 ISBN 0060649526, 9780060649524 
- Monarchians, New Advent, Catholic Encyclopedia
- Tertullian, Against Praxeas, III, c.213
- Views of Sabellius, The Biblical Repository and Classical Review, American Biblical Repository
- Select Treatises of St. Athanasius - In Controversy With the Arians - Freely Translated by John Henry Cardinal Newmann - Longmans, Green, and Co., 1911, footnote, page 124
- Creeds of the Catholic Church
- Servetus, Swedenborg and the Nature of God by Andrew M.T. Dibb, Lanham, Maryland: University Press of America Inc, 2005