'Heir' in the Bible

Now the promises [in the covenants] were decreed to Abraham and to his seed. God does not say, “And to seeds (descendants, heirs),” as if [referring] to many [persons], but as to one, “And to your Seed,” who is [none other than] Christ.

Why, then, the Law [what was its purpose]? It was added [after the promise to Abraham, to reveal to people their guilt] because of transgressions [that is, to make people conscious of the sinfulness of sin], and [the Law] was ordained through angels and delivered to Israel by the hand of a mediator [Moses, the mediator between God and Israel, to be in effect] until the Seed would come to whom the promise had been made.

Now what I mean [when I talk about children and their guardians] is this: as long as the heir is a child, he does not differ at all from a slave even though he is the [future owner and] master of all [the estate];

Therefore you are no longer a slave (bond-servant), but a son; and if a son, then also an heir through [the gracious act of] God [through Christ].

But what does the Scripture say?“Cast out the bondwoman [Hagar] and her son [Ishmael],For never shall the son of the bondwoman be heir and share the inheritance with the son of the free woman.”