“Therefore you are no longer a slave but a son, and if a son, then an heir of God through Christ.”
Therefore
Paul concludes his argument about sonship with a summary of his argument by the word “therefore.”
you
The word “you” is singular, making the application very personal to the Galatians.
are no longer a slave but a son,
The believer is not a slave under the law but a son with adult privileges with God under grace. The Christian is free from the law because he is a “son.” Sonship brings with it the claims of an heir. The Galatians were free from slavery to the law but stood under grace before God.
and if a son, then an heir of God through Christ
We are heirs of God and joint heirs with Christ.
“The Spirit Himself bears witness with our spirit that we are children of God, and if children, then heirs—heirs of God and joint-heirs with Christ, if indeed we suffer with Him, that we may also be glorified together” (Romans 8:16-17).
Principle:
Because we are God’s sons, we are heirs to His provisions.
Application:
Christians are joint heirs with Christ. Many married couples have joint bank accounts. Often, the first thing young couples do after getting married is open a joint account. They put all their assets into the account in one lump sum. They assume it is a 50/50 account; 50 percent is hers, and 50 percent is his. It doesn’t work that way, however. She beats him to the withdrawal every time. Our joint account with Christ is not a 50/50 deal. We hold all of His assets equally with Him. Are you writing spiritual checks that are tantamount to the kind of account you have with God?
What Jesus Christ did for us in the incarnation was incalculable. The Son of God became the Son of Man that the sons of men might become the sons of God. Christ came down from where He was to where we are so that He might lift us from where we are to where He is eternally.
Christ is the heir of all things (Hebrews 1:1,2). His privilege as an heir is based on His Sonship (Romans 8:16,17); He is an heir because He is the Son of the Father. We are heirs of Christ and fall heirs to eternal life (Titus 3:7); to fall heir to Christ demands that we fall heirs to eternal life. The heirs of Christ share His destiny (Ephesians 1:11). The Father elected Christ and us from eternity (Hebrews 9:15). Therefore, we share Jesus’ election and destiny. This guarantees our eternal security (1 Peter 1:4, 5).
God always makes us heirs based on His grace (Galatians 3:29). The Holy Spirit is the down payment on our eternal inheritance (Galatians 4:6; Ephesians 1:14).

Dear Dr. Richison,
I read Galatians 4: 1-7 today and found it such a blessing. Your commentary on the adoption of sons, the sending forth of the Spirit, and an heir of God helps me to appreciate the wonderful grace and love of God in my life.
May God bless you as you continue to explain His Word.
Thank you Eunice.