Adoptionism (also called Dynamic Monarchianism), is a subset of non-Trinitarianism that holds that Jesus was a mere mortal man, and that he was “adopted” as God’s Son at some point in his earthly life, usually at his baptism. The synods of Antioch and Nicea rejected Adoptionism, and it is a sub-Christian heresy.
Theodotus of Byzantium in the second century is the first known proponent of this view. Paul of Samosata in the third century followed Theodotus in this view. Certain aspects of this heresy arose in the 8th and 9th century in Iberia.
Scholar Bart Ehrman has popularized the idea that early Christians were adoptionists, pointing out that “son of God” is not present in certain early manuscripts of Mark until Christ’s baptism. Others defend this view by pointing out that the Apostle Paul does not mention the virgin birth of Christ in his epistles.
Other Adoptionists include some Unitarians and certain Mormons, led by LDS leader, James Strang, who was convinced that Jesus became God through the process of Eternal Progression.