Adam Clarke on Galatians 4:7

Thou art no more a servant - Thou who hast believed in Christ art no longer a slave, either under the dominion of sin or under obligation to the Mosaic ritual; but a son of God, adopted into the heavenly family.

And if a son, then an heir - Having a right to the inheritance, because one of the family, for none can inherit but the children; but this heirship is the most extraordinary of all: it is not an heirship of any tangible possession, either in heaven or earth; it is not to possess a part or even the whole of either, it is to possess Him who made all things; not God's works, but God himself: heirs of God through Christ.