In chapter 3, Paul had introduced the idea of eternal life as a holy inheritance from our Father Who is in heaven.
This was according to the Will and Testament of the Father, and there was nothing anything that came after God’s Will was given which could annul His will for those who are descendants of the seed of Abraham – Who is the Lord Jesus Christ.
We become sons of God when we place our faith and trust in the promise of God pertaining to His Son, in the same way Abraham placed his faith and trust in what God about His Son. Abraham believed by faith in Jesus Christ, and it was accounted to him for righteousness in the same way it accounted to us for righteousness when we likewise believe in Jesus Christ.
Only those who place their faith in the Lord Jesus Christ are sons of Abraham. God’s Will states all those who are sons of Abraham by faith in the Lord Jesus Christ shall receive eternal life in the kingdom of heaven – which is the holy inheritance spoken of in God’s Will and Testament. The Law of God, which came 430 years later, cannot annul God’s Will and Testament for the sons of Abraham.
In chapter 4, Paul reveals the distinction between those who are in the household who are sons of God and those who are sons of the slaves of The Law.
This is a brilliant illustration. (Basically all of chapter 4 is illustration, and it demonstrates how effective the proper use of illustration can be.)
Paul says when children are present in the home you can’t tell which children are children of the master of the house, and which children are children of the slaves who serve the master. It isn’t readily apparent. They are all present in the master’s house together, and all kept under the care of the guardians and stewards of the master’s home.
Both children of the master and children of the slave are held under bondage and are alike until the fullness of time, when maturity takes place. At this point, it becomes obvious which children are the children of the master, and which children are children of slaves.
Paul is being patently obvious here. You do not want to be found children of slavery, and you would never make that choice if it was offered to you. To the Galatians Paul is saying this is the choice you are making when you choose to go back and seek personal righteousness by following the Law of God rather than the Spirit of God.
It is demonstrated by your newfound slavery to the Law of God you had been born again completely free of. Don’t go back. Never go back. There is no holy inheritance for the children of the slaves, but only for children of the free.