Second-century Christian heresy
which took two forms, Dynamic and Modalistic.
The orthodox position regarding Christ's nature is that he is the Son of God, eternally begotten of the Father, a Person of the Trinity, and as Jesus of Nazareth he was both fully God and fully man. Monarchianism opposed such Trinitarianism, seeing it as dangerously close to polytheism.
Dynamic Monarchianism held that Jesus of Nazareth was an ordinary man in whom dwelled a force that issued from God, and therefore he partook of God's power but not his nature or personality. Some, called Adoptionists, taught that this power entered Jesus either at his baptism or his resurrection, making him an adopted son of God.
Modalistic Monarchianists believed that the three "Persons" of the Trinity were actually aspects of God, masks that He used when dealing with humankind. In the end, when these masks are no longer necessary, the characters of "Father", "Son" and the "Holy Spirit" will be put aside and God will be perceived as a single and undifferentiated being.
This heresy persists even today, though it is unlikely that its adherents would refer to themselves as Monarchianists.* As the concept of the Trinity is notoriously difficult to grasp, they may not even be aware that their beliefs are in any way unorthodox. Modalistic Monarchianism has also been known as Patripassionism, Sabellianism, and Noeticism.
*A character in Tom Robbins' novel Another Roadside Attraction expresses the Adoptionist wish that the Church would portray Jesus was just an ordinary bum until God adopted him at the crucifixion. That way, he says, people might be nicer to one another since you never know who God might decide to pick out as His own.