Monarchianism is a set of beliefs that emphasize God as being one person, in direct contrast to Trinitarianism which defines God as three persons coexisting consubstantially as one in being.
Various models of resolving the relationship between God the Father and the Son of God were proposed in the 2nd century, but later rejected in favor of the doctrine of the Trinity as expounded at the First Council of Constantinople, which confirmed the concept of God as one being consisting of three persons: the Father, the Son, and the Holy Spirit.
Monarchianism was generally credited to Paul of Samosata, a bishop of Antioch.
Two contradictory models of monarchianism have been propounded:
Both schools of Monarchians found a strong wall of opposition to them elevated very quickly in the form of the Logos theologians (Tertullian, Hippolytus, Clement of Alexandria, and Origen of Alexandria).
The name Monarchian properly does not strictly apply to the Adoptionists, or Dynamists, as they (i.e., the latter) "did not start from the monarchy of God, and their [doctrine] is strictly Christological".