Do Reformed Christians really believe in adoption?

Not the doctrine but the practical reality of how it is lived out in the Body of Christ. Ask a solidly Reformed believer about the subject and they can expound on the idea of adoption in theology. Ask us how that should impact how we view one another and you are less likely to get a quick and smooth answer because I don’t think we apply this doctrine terribly well. If there is a glaring weak spot in Reformed theological thought, it is putting the system of theology into practice. So much of the practical reality of Reformed folks is spent on stuff that isn't really all that Reformed and really is little more than church traditions. At the same time we erect barriers to fellowship with those who are brothers and sisters. Is that Scriptural? Are we not people who cry sola scriptura?

Let's look at a couple of passages:

But he replied to the man who told him, "Who is my mother, and who are my brothers?" And stretching out his hand toward his disciples, he said, "Here are my mother and my brothers! For whoever does the will of my Father in heaven is my brother and sister and mother." (Mat 12:48-50)

For all who are led by the Spirit of God are sons of God. For you did not receive the spirit of slavery to fall back into fear, but you have received the Spirit of adoption as sons, by whom we cry, "Abba! Father!" The Spirit himself bears witness with our spirit that we are children of God, and if children, then heirs--heirs of God and fellow heirs with Christ, provided we suffer with him in order that we may also be glorified with him. (Rom 8:14-17)

Know then that it is those of faith who are the sons of Abraham. (Gal 3:7)

For it was fitting that he, for whom and by whom all things exist, in bringing many sons to glory, should make the founder of their salvation perfect through suffering. (Heb 2:10)

But when the fullness of time had come, God sent forth his Son, born of woman, born under the law, to redeem those who were under the law, so that we might receive adoption as sons. And because you are sons, God has sent the Spirit of his Son into our hearts, crying, "Abba! Father!" So you are no longer a slave, but a son, and if a son, then an heir through God. (Gal 4:4-7)

Blessed be the God and Father of our Lord Jesus Christ, who has blessed us in Christ with every spiritual blessing in the heavenly places, even as he chose us in him before the foundation of the world, that we should be holy and blameless before him. In love he predestined us for adoption as sons through Jesus Christ, according to the purpose of his will, to the praise of his glorious grace, with which he has blessed us in the Beloved. (Eph 1:3-6)

So it is clear that our brothers and sisters, adopted into the family of God, are our fellow believers. Not only that, but as Ephesians 1 tells us, we were predestined to that relationship. God ordained and planned that relationship before the foundation of the earth, and His Son died on a cross to pay for their sins and bring them into familial relationship with God and one another. The practical reality is that most of the time we go out of our way to separate ourselves from one another, rejecting those from fellowship who fail to subscribe to every church practice or who are not "members" or who are outside of our denominations. Is there a place for gathering with like minded believers? I think so. Is that a excuse to reject all fellowship with others who have been adopted in the family of God? No.

We who are Reformed should be the least likely of all people to erect barriers to fellowship. For those who believe falsely in a “free will” arminianism, the Body of Christ is a voluntary association that you joined of your own choice, initiative and volition. Why not then choose to exclude some and be in fellowship with certain others? After all the choice is yours. But for those who believe that God sovereignly chose us, predestined us for adoption into His family before the foundation of the world to be sons and daughter, how do we justify partitioning ourselves? We did not choose the Body, Christ the head of the Body chose us. God has chosen and created and ordained His family of which we are members. Since we had nothing to do with the creation of that family, how do we exclude so many of those that God has also chosen as our brothers and sisters? Those people over there? They are your brothers and sisters. That group over there? Also your brothers and sisters. We were quickened by the same spirit, our sins were propitiated on the same cross, we were justified by faith in the same Savior.

God has called us to be brothers and sisters in His Body. We have decided that some of us are OK to be in fellowship with and others of us are not. Sure we might gather at the occasional conference or two, T4G or Gospel Coalition, where we intentionally and openly recognize that we are temporarily setting aside differences. But once the conference is over, we go our own way, back to our "churches" and behind our walls.

If the Reformed camp has a weakness (other than being a camp at all), it is in putting our theology to practice. Adoption is one of those places where we say and believe one thing, but our practice is not that reflective of it. Every one of God's regenerate elect is our brother and sister, and we should not put them outside of the camp and save fellowship with them for eternity. It is not Biblical and it is not Reformed to do so.