Galatians 4:7 - Matthew Poole's English Annotations on the Holy Bible

Thou that art a believing Gentile, as well as the believing Israelites, art no more a servant, not in that state of servile subjection to the law; but a son; but in a more excellent state of liberty, like unto that of sons that have attained to a full and ripe age. Christ told his disciples, 1 Thessalonians 15:15, that he did not call them servants, for servants knew not what their lord did; but he had freely communicated to them what he had received from the Father. The apostle here saith, they were sons, sons by adoption; which is the highest notion of freedom and liberty. And this entitled them to an inheritance: if a son, then an heir of God through Christ: which agreeth with Romans 8:17. And as it is with sons and heirs, though the inheritance cometh not fully to them till the death of the parent, yet while they live they are in a far better condition than servants; so the believing Gentiles, being made sons and heirs of God through Christ, though they were to stay a while for the inheritance reserved for the sons of God in the heavens, yet their state was much better than that of servants; for though they were obliged to serve the Lord, yet they served him without servile fear, and were no otherwise servants than sons are also servants to their father.