Monarchianism (mono--"one," arche--"rule") was an error concerning the nature of God that developed in the second century A.D. It arose as an attempt to maintain Monotheism and refute tritheism. Unfortunately, it also contradicts the orthodox doctrine of the Trinity. Monarchianism teaches that there is one God as one person: the Father. The Trinity is that there is one God in three persons: Father, Son, and the Holy Spirit. The Trinity is monotheistic and not polytheistic as some of its critics like to assert. Monarchians were divided into two main groups: the dynamic monarchians and the modal monarchians.
Dynamic Monarchianism teaches that God is the Father and that Jesus is only a man, denied the personal subsistence of the Logos, and taught that the Holy Spirit was a force or presence of God the Father. Present-day groups in this category are the Jehovah's Witnesses, Christadelphians, and Unitarians. Additionally, some ancient dynamic monarchianists were also known as Adoptionists who taught that Jesus was tested by God, and after passing this test and upon His baptism, He was granted supernatural powers by God and adopted as the Son. Ancient teachers of dynamic monarchianism were Theodotians, a Tanner in Byzantium around A.D. 190, and Paul of Samosata a bishop of Antioch in Syria around A.D. 260.
Modal monarchianism teaches that the Father, the Son, and the Holy Spirit are just modes of the single person who is God. In other words, the Father, Son, and Holy Spirit are not simultaneous and separate persons but consecutive modes of one person. Praxeas, a priest from Asia Minor, taught this in Rome around A.D. 200. Modern groups in this general category are the Oneness Pentecostal groups known as the United Pentecostal and United Apostolic Churches. However, the present-day modalists maintain that God's name is Jesus. They also require baptism "in Jesus' name" not "in the name of the Father, the Son, and the Holy Spirit" for salvation.