What is adoptionism? Why is it a dangerous belief to hold to?
Adopted by God
Before we discuss what adoptionism is, we need to know about biblical adoption, and that is God adopting us. The Apostle Paul explains that “he chose us in him before the foundation of the world, that we should be holy and blameless before him. In love” (Eph 1:4), so the motive was simply love. It was not that God saw us and thought we were special or a cut above the others. No, He decided to save us…in love. It was in His love that He “predestined us for adoption to himself as sons through Jesus Christ, according to the purpose of his will” (Eph 1:5). Since we are now adopted by God, that means “In him we have obtained an inheritance, having been predestined according to the purpose of him who works all things according to the counsel of his will” (Eph 1:11). Believers love God, but it was God Who made the first move, because “We love because he first loved us” (1 John 4:19). Put another way, we love God only because He loved us first. Being the children of God is not a future event, but rather, “we are God’s children now, and what we will be has not yet appeared; but we know that when he appears we shall be like him, because we shall see him as he is” (1 John 3:2), so what about Jesus and the teaching going around that He was adopted by God the Father? That is a very dangerous heresy and it’s called adoptionism, and we’ll examine it next.
Adoptionism is different from modalism which portends that Father and Son are two historical roles, but of a single divine Person. This is contrary to the biblical doctrine of the Trinity, where each of the Three Persons of the God head is identified in Scripture. All Three are distinguished from one another, and yet they are at one in essence and unity. God the Father is God and always has been. Jesus Christ is God and always has been. And the Holy Spirit is not a force or power, but a Person, and He is equally God. Adoptionism is the belief that Jesus was God the Father’s adopted Son. He was adopted because in His humanity, He never sinned, but this is sheer heresy because it teaches that, at one point, Jesus was not God but became God. That, my friend, is contrary to Scripture. For example, John 1:1 states that “In the beginning was the Word, and the Word was with God, and the Word was God.” Clearly, Jesus is the Word, “And the Word became flesh and dwelt among us, and we have seen his glory, glory as of the only Son from the Father, full of grace and truth” (John 1:14). The Apostle John was an eye witness to Jesus’ humanity and His glory. And even more, Jesus “was in the beginning with God” (John 1:2), and remember, no man can see God and live (Ex 33:17-33). And finally, since “All things were made through him, and without him was not any thing made that was made” (John 1:3), this means Jesus existed before the creation, and in fact, He was the Creator. Since He is the Creator of all things (Heb 1:2), and man is a created being, Jesus could never have been just a man who lived a perfect life and then became God because of that. By the way, as the Creator, He could not have created Himself! That’s ridiculous because it’s impossible to be a created being and yet be the Creator, and so adoptionism is utter heresy straight from the mouth of Satan.
Jesus is God
Adoptionism is wrong in many places, and that includes the belief that Jesus became God. The Bible teaches that Jesus was both Man and God, but mankind is born with a sinful nature, and all men sin (Rom 3:10-12, 23, 5:12-14), so if Jesus was originally only human, then Jesus would have had a sinful nature, and we know He did not (2 Cor 5:20-21)! And we cannot be redeemed by the death of a human. Our own human blood can’t even cover our own sins, not to mention the sins of others. It is only “with the precious blood of Christ, like that of a lamb without blemish or spot” (1 Pet 1:19) that we can be cleansed, and no man is unblemished. Yes, Jesus is “the only Son from the Father” (John 1:14), but the Holy Spirit is the only Holy Spirit that exists, and of course, there’s the one and only Father. The point being, this doesn’t mean Jesus became a Son. He has always been the Son of God. Adoptionism teaches that Jesus tried really, really hard and didn’t sin, so He qualified to become a god, but there are cults that teach the same thing, but it’s not from God. Humans will never become gods. There is only the God the Father, God the Son, and God the Holy Spirit. We can receive eternal life, and become the children of God, but we cannot “qualify” to become God as God is. You cannot become God anyway, because by definition, God has always existed and always will. We can’t say that about us. God is infinite. We are finite! It’s not even close!
I would rather see us teach how we can become more like Jesus. We’ll never become god or be exactly like God, but we can be more like Jesus. The Apostle Paul tells us, “Have this mind among yourselves, which is yours in Christ Jesus, who, though he was in the form of God, did not count equality with God a thing to be grasped” (Phil 2:2-6). Jesus as God in all His glory left heaven to be born as a babe in humble circumstances and then die on the cross in shame and agony. He didn’t try to grasp or hold onto His rights as God, but He thought of us as He “emptied himself, by taking the form of a servant, being born in the likeness of men. And being found in human form, he humbled himself by becoming obedient to the point of death, even death on a cross” (Phil 2:8-9). That’s the kind of mind I want to adopt my thinking into. Knowing just how much Christ gave up and suffered should compel us to “Do nothing from selfish ambition or conceit, but in humility count others more significant than yourselves. Let each of you look not only to his own interests, but also to the interests of others” (Phil 2:3-4). We can become more like Jesus, but we can never become Jesus, but that’s the kind of thinking that adoptionism teaches!
Be on the watch for this false teaching called adoptionism. It may vary in form or be a mutation of some other heresy, so we must “contend for the faith that was once for all delivered to the saints” (Jude 1:3b). There will be no new deliveries of the faith, of that you can be sure! God has already spoken through His Son and the apostles and prophets (Heb 1:1-2). We must be diligent because “certain people have crept in unnoticed who long ago were designated for this condemnation, ungodly people, who pervert the grace of our God into sensuality and deny our only Master and Lord, Jesus Christ” (Jude 1:4). Jude warns, “It is these who cause divisions, worldly people, devoid of the Spirit” (Jude 1:19), and without the Spirit of God, they cannot truly know the Son of God. Jesus said “when the Helper comes, whom I will send to you from the Father, the Spirit of truth, who proceeds from the Father, he will bear witness about me” (John 15:26). The Spirit of God and the Word of God will be your best defense from the heresies about God, including those about Jesus. And this would include adoptionism.
Here is some related reading for you: What is a Heretic? Are there Heretics in the Church? 
Resource – Scripture quotations are from The Holy Bible, English Standard Version® (ESV®), Crossway Bibles. (2007). ESV: Study Bible: English standard version. Wheaton, Ill: Crossway Bibles. Used by permission. All rights reserved.