In Christianity, Sabellianism is the Western Church equivalent to Patripassianism in the Eastern Church, which are both forms of theological modalism. Condemned as heresy, Modalism is the belief that the Father, Son and Holy Spirit are three different modes of God, as opposed to a Trinitarian view of three distinct persons within the Godhead. However, Von Mosheim, German Lutheran theologian who founded the pragmatic school of church historians, argues that Sabellius "believed the distinction of Father, Son, and Holy Spirit, described in the Scriptures, to be a real distinction, and not a mere appellative or nominal one."

The term Sabellianism comes from Sabellius, who was a theologian and priest from the 3rd century. None of his writings have survived and so all that is known about him comes from his opponents. The majority believe that Sabellius held Jesus to be deity while denying the plurality of persons in God and holding a belief similar to modalistic monarchianism. While Sabellius did maintain that only one divine Person existed, he used the word person as synonym for nature:

"Sabellius held to the simple unity of the person and nature of God."

Since the distinction between ousia (substance) and hypostasis (person) (both of which mean ‘something that subsists’) was worked out only in the late fourth century, Sabellius used the word person in a different sense. But Sabellius did describe God as three in one sense but one in another. Modalistic monarchianism has been generally understood to have arisen during the 2nd and 3rd centuries, and to have been regarded as heresy after the 4th, although this is disputed by some.

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 of one substance by the Athanasian Creed, probably dating from the late 5th or early 6th century. The Greek term homoousian or (ὁμοούσιος 'consubstantial') had been used before its adoption by the First Council of Nicaea. The Gnostics were the first to use the word ὁμοούσιος, while before the Gnostics there is no trace at all of its existence. The early church theologians were probably aware of this concept, and thus of the doctrine of emanation, taught by the Gnostics. In Gnostic texts the word ὁμοούσιος is used with the following meanings:

• Identity of substance between generator and generated.
• Identity of substance between things generated of the same substance.
• Identity of substance between the partners of a syzygy.

The term ὁμοούσιος was already in current use by the 2nd-century Gnostics, and through their works it became known to the orthodox heresiologists, though this Gnostic use of the term had no reference to the specific relationship between Father and Son, as is the case in the Nicene Creed. It has been noted that this Greek term homoousian ('same being' or 'consubstantial'), which Athanasius of Alexandria favoured, was also a term reportedly used by Sabellius—a term that many who held with 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, operating as different manifestations or modes. Athanasius' use of the word is intended to affirm that while the Father and Son are eternally distinct in a truly personal manner (i.e. with mutual love, per John 3:35, 14:31), both are nevertheless one being, essence, nature, or substance, having one personal spirit.