Trinity Evangelical Lutheran Church, Creston, Iowa;
Now before faith came, we were held captive under the law, imprisoned until the coming faith would be revealed. So then, the law was our guardian until Christ came, in order that we might be justified by faith. But now that faith has come, we are no longer under a guardian, for in Christ Jesus you are all sons of God, through faith. For as many of you as were baptized into Christ have put on Christ. There is neither Jew nor Greek, there is neither slave nor free, there is no male and female, for you are all one in Christ Jesus. And if you are Christ’s, then you are Abraham’s offspring, heirs according to promise. I mean that the heir, as long as he is a child, is no different from a slave, though he is the owner of everything, but he is under guardians and managers until the date set by his father. In the same way we also, when we were children, were enslaved to the elementary principles of the world. 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.” (Galatians 3:23–4:7, ESV)
Grace and peace to you from our Lord and Savior, Jesus Christ.
St. Paul uses the metaphor of adoption. It may not be exactly what you think. In ancient Rome, the context of Paul's letter to the Galatians, the heirs of the household were adopted by their parents to receive the inheritance. Before being adopted they were under the control of the guardian. This guardian or pedagogue was a servant who is entrusted with the care and especially protection of the child who would become the heir. They would oversee the comings and goings of the child. They would watch over everything the child did. The child was not free but under the control of the pedagogue. Then, at the discretion of his father, a date would be set for the child to "come of age". The age was usually somewhere between 15 and 18. As Paul says "he is under guardians and managers until the date set by his father" (4:2). On that date the child was officially adopted by the father and was formally recognized as the son and then received full inheritance rights. Until that date the child was no different in the eyes of the law than a slave. He had no freedom and no power to make decisions. But on the date of his adoption, the date set by his father, all that changed. He had the full responsibility and freedom of the heir.
The reason that St. Paul uses this metaphor is because the Galatian church, the recipients of this letter, had fallen into the trap of accepting the teachings of the Judaizers. These false teachers had convinced the people of the congregation that in order to become "heirs" more was necessary then only faith. They falsely taught that there was the necessity of becoming circumcised, following the dietary laws of the Old Testament, and doing and not doing certain things. Unless these were done one could not be a "true" Christian. This is the false teaching of adding our works to God's grace. It is the danger the church must always be on guard against.
Paul lays out the truth in clear and certain terms. You are justified only through faith in Jesus Christ. Through faith we receive God's promised inheritance. The promise is "for you and your children". There is no male or female, slave or free but all are one in Christ.
That's when Paul uses the metaphor of the pedagogue. He says this is what the law does. Those who are under the law have their freedom restricted by the law. They are constrained by the custodian. They are no different "then a slave". However, when we are "in Christ", our status is changed. The Father has set the date. "In the fullness of time". It is the time of Jesus. The time when God sent his son into the world to redeem those under the law. This is what Jesus does for you when you were under the law. He removes the restrictions and the punishments for disobedience. God, in human flesh, is born of a virgin so that the law applies to him. As a human being he is required to keep the law perfectly or suffer the just punishment of God for disobedience. This is what it means to be under the law. Jesus is therefore under the law. And yet, he does not break the law but fulfills it. He keeps it perfectly in every respect. Jesus lives in a perfect relationship with God the Father. And he lives in perfect relationship with his fellow human beings. And then he is sent to the cross to receive the punishment that is deserved for breaking the law, even though he did not break it. This is what Jesus does in "the fullness of time". He lives a perfect life so that it may be given to you for you to put on in Holy Baptism. And he takes the punishment for your sin setting you free from the curse of the law.
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.” (Galatians 4:4–5, ESV)
Jesus makes you the heir. You are adopted by God the Father. You have full rights of inheritance. That is exactly what Paul talks about when he speaks about your adoption. And make no mistake when St. Paul talks about adoption into faith he is speaking about Holy Baptism. When he speaks about being in Christ he is pointing to what happens in Holy Baptism. Martin Luther agrees:
What benefits does Baptism give?
It works forgiveness of sins, rescues from death and the devil, and gives eternal salvation to all who believe this, as the words and promises of God declare. (From Luther’s Small Catechism © 1986 Concordia Publishing House)
It is in Holy Baptism that the "fullness of time" that was brought by Jesus comes to you. Your adoption happens when water is poured on your head along with God's name. And all that Jesus did in the "fullness of time" is yours. Luther makes the point that the most important words in baptism are "for you". This is what it means to "put on Christ". Listen to Martin Luther and the large catechism:
In this way one sees what a great, excellent thing Baptism is. It delivers us from the devil’s jaws and makes us God’s own. It suppresses and takes away sin and then daily strengthens the new man. It is working and always continues working until we pass from this estate of misery to eternal glory. For this reason let everyone value his Baptism as a daily dress [Galatians 3:27] in which he is to walk constantly. Then he may ever be found in the faith and its fruit, so that he may suppress the old man and grow up in the new. 
And what a privilege we have today to see such a great picture of the inheritance in Jesus Christ. Almost a whole family young and old (well old-er). It's an adoption of sons. God is taken these four men to himself. They have put on Christ. They are full heirs of God's promises. From this day forward they will live "in Christ". And through faith in what Jesus has done for them here today they will join us with Jesus forever.
And so we are all today, one in Christ. There is neither slave nor free, black or white, male or female, young or old, rich or poor. We are one because we are in Christ. We have put on Christ's righteousness. That is all that he did in his perfect life is ours. We are free to live that way. Free from the worry of punishment for our sin even though our sin plagues us every day. Jesus death on the cross removes our punishment for it. And so, "in Christ", we walk constantly in the daily dress of Jesus Christ. Growing up in him to be like him every day. Doing the things that God would have us do in help to our neighbors. And every day living in the forgiveness that he won for us. Every day knowing that we are indeed heirs of eternal life. Amen.
The peace of God that passes all understanding, keep your hearts and minds in Christ Jesus. Amen.
 Concordia: The Lutheran Confessions. 2005 (P. T. McCain, Ed.) (431). St. Louis, MO: Concordia Publishing House.