Adoptionism is an error concerning Christ that first appeared in the second century. Those who held it denied the preexistence of Christ and, therefore, His deity. Adoptionists 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. As a reward for His great accomplishments and perfect character, Jesus was raised from the dead and adopted into the Godhead.
This error arose out of an attempt by people to understand the two natures of Jesus. The Scriptures tell us that Jesus is both God and man: "for Him dwells all the fullness of deity in bodily form," (Col. 2:9). This is known as the doctrine of the Hypostatic Union where in the one person of Christ, there are two natures: God and man.
Theodotus of Byzantium was the most prominent adherent to this error.
Adoptionism was condemned as a heresy by Pope Victor (A.D. 190-198).
8th Century revision
Adoptionism was later revived in the 8th Century in Spain by Elipandus, archbishop of Toledo, and Felix, bishop of Urgel. This was a variation of the first error, but it held that Christ was the Son of God in respect to His divine nature but that as a man, He was only adopted as the first born of God.
In 798 Pope Leo III held a council at Rome that condemned adoptionism as a heresy.