To redeem them that were under the law, that we might receive the adoption of sons, Galatians 4:5.
In order to understand adoption, we need to back up a little and look at another doctrine that runs hand in hand with adoption, imputation. One of the things that imputed righteousness and adoption have in common is that they are both future events absolutely guaranteed to the saint of God. It is popular to say that we have imputed righteousness just as Abraham did, but we do not. The Apostle Paul puts our imputed righteousness in the future.
Romans 4:23 Now it was not written for his sake alone, that it was imputed to him;
Romans 4:24 But for us also, to whom it shall be imputed, if we believe on him that raised up Jesus our Lord from the dead;
For a moment, try to forget your indoctrination and try to focus on what Paul is saying. Start by getting the context. Abraham had a body, a soul and a spirit. Romans Chapter 4 is not about the complete man. It speaks of his flesh. What shall we say then that Abraham our father, as pertaining to the flesh, hath found?, Romans 4:1. The Apostle Paul went to great lengths to teach us. Part of that teaching is to explain how God handles the problem of sinful flesh.
We know that God can change a heart, and we know that he can give a man another spirit. What he does not do here on this earth is give a man a new body. What God did for Abraham was to impute God's righteousness to Abraham's flesh. When Abraham walked on this earth, God treated his flesh as if it was God's own flesh. He poured wealth and honor upon it. He made promises to its offspring.
This is not the status of the Christian. A Christian will have imputed righteousness to his flesh, but he does not have it now. A Christian does not need imputed righteousness to get into heaven, His inward man is the righteousness of God. For he hath made him to be sin for us, who knew no sin; that we might be made the righteousness of God in him, 2nd Corinthians 5:21. The inward man of a Christian is perfectly righteous. In fact, it is as righteous as God himself. It is the righteousness of God. It is no wonder that we are called new creatures in Christ.
It is in the process of adoption that God imputes righteousness to the flesh of a Christian. My inward man is born of God. We are told that it cannot sin, and he cannot sin, because he is born of God, 1st John 3:9. My flesh has a problem. It cannot stop sinning. It cannot be born of God as my inward man is. My flesh was born on April 2, 1954 in Salamanca, New York. Its father is a man who was a sinner just as I am. In order for my flesh to be allowed into heaven it needs two things. It needs the righteousness of God imputed to it, and it needs God the Father to adopt it as his own. That is exactly what the good apostle promised it when he said, to whom it shall be imputed, and it is the gist of Romans chapter 8.
Read Romans 8:23. In plain English it tells you that we are waiting for the adoption and it tells us that the adoption is the redemption of our body. And not only they, but ourselves also, which have the firstfruits of the Spirit, even we ourselves groan within ourselves, waiting for the adoption, to wit, the redemption of our body, Romans 8:23. If you have salvation you are not adopted. You shall be adopted. God has given you the Spirit of adoption. That Spirit is the earnest of your inheritance. It is God's proof inside of the believer that God will redeem our bodies.
Ephesians 1:13 In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise,
Ephesians 1:14 Which is the earnest of our inheritance until the redemption of the purchased possession, unto the praise of his glory.
The redemption of the purchased possession is the rapture. It is the adoption of our bodies by God the Father after he imputes righteousness to our flesh. If you have salvation that is a sure thing.