Therefore you are no more a servant, but a son; and if a son, then an heir of God through Christ.
Read Chapter 4
Ambrosiaster
AD 400
There is no doubt that one adopts a son in order to leave him an heir; but inheritance depends on the death of someone. How then can mortals be called the heirs of him who lives forever? The fact is that Scripture speaks in our own manner so that we may understand. In order to show that the Father will give from his goods those things that he is going to give his sons, it calls this “inheritance.”
See how He has admitted those to be children who are under fear and sins; but has conferred manhood on those who are under faith, by calling them sons, in contradistinction from the children that are under the law: "For thou art no more a servant "he says, "but a son; and if a son, then an heir through God."
What we say in this place we should also observe in others, that the whole human race is being treated under a single term. For all we who believe are one in Christ Jesus and members of his body. .