Heresy is any belief or practice that explicitly undermines the gospel and can easily be proven false through scripture alone.
This heresy first appeared approximately 200 years after Jesus' resurrection. Let's break this down into its fundamental beliefs and see what the scriptures say about it and why it is heresy and completely false.
It denies the preexistence of Christ and, therefore, His deity.
Adoptionists taught that God tested Jesus, and after passing this test and upon baptism, He was granted supernatural powers by God and adopted as the Son.
They also taught that Jesus was raised from the dead and adopted into the Godhead as a reward for His great accomplishments and perfect character.
These false beliefs more than likely occurred because people were trying to understand the two natures of Jesus. Scriptures tell us Jesus is both man and God. (Colossians 2:9).
This scriptural doctrine of one person (Christ) having both natures (God and man) is called the Hypostatic Union.
Pre-existence of Christ proof-text
John 8:23; 58-59
1 John 1:2
Matthew 28:9, 17
Ephesians 2:14-15 and many more.
Deity of Jesus
1 John 5:20
John 5:23 as well as others.
As far as points #2 and #3, there is zero scriptural support and no evidence whatsoever of these two beliefs.
Pope Victor condemned Adoptionism as a heresy (190-198 A.D.).
Adoptionism was revised in the 8th century by the Archbishop Elipandus of Toledo in Spain and Bishop Felix 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 firstborn of God.
Pope Leo III held a council at Rome in 798 A.D. that again condemned Adoptionism as heresy.