Sabellius (fl. ca. 215) was a third-century priest and theologian who most likely taught in Rome, but may have been a North African from Libya. Basil and others call him a Libyan from Pentapolis, but this seems to rest on the fact that Pentapolis was a place where the teachings of Sabellius thrived, according to Dionysius of Alexandria, c. 260. What is known of Sabellius is drawn mostly from the polemical writings of his opponents.

The Catholic Encyclopedia writes:

It is true that it is easy to suppose Tertullian and Hippolytus to have misrepresented the opinions of their opponents, but it cannot be proved that Cleomenes was not a follower of the heretical Noetus, and that Sabellius did not issue from his school; further, it is not obvious that Tertullian would attack Callistus under a nickname.

Sabellius' opposition to the idea of the Trinity led to his excommunication as a heretic by Callixtus in AD 220. Wace and Bunsen have both suggested that Calixtus' action was motivated more by a desire for unity rather than by conviction.

Sabellius taught that God was single and indivisible, with Father, Son, and Holy Spirit being three modes or manifestations of one divine Person.

This understanding has been called Sabellianism or modalistic monarchianism. A Sabellian modalist would say that the One God successively revealed Himself to man throughout time as the Father in Creation; the Son in Redemption; and the Spirit in Sanctification and Regeneration. (Because of this focus on God's revelation of himself to man, Modalism is often confused with economic Trinitarianism).

It has been noted also that the Greek term "homoousios", which Athanasius of Alexandria favored, was actually a term that was reported to be put forth and favored also by Sabellius, and was a term that many followers of Athanasius took issue with and were uneasy about. Their objection to the term "homoousios" was that it was considered to be "un-Scriptural, suspicious, and of a Sabellian tendency." Athanasius, however, used the term differently than Sabellius, affirming oneness of the Divine Essence while maintaining the distinctions between the Divine Persons.

The term "homoousios" was accepted, however, at the Council of Nicaea in 325 A.D., using the Athanasian formula and doctrine, of the Father and Son being distinct persons, though co-eternal, co-equal, and con-substantial. The objections to using the term were addressed by clarifying that it was not being used in the Sabellian sense of oneness of Person, but rather to denote oneness of Essence while affirming the distinctions of the Persons or "hypostases".