'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 I say, That the heir, as long as he is a child, differeth nothing from a servant, though he be lord of all;

Wherefore thou art no more a servant, but a son; and if a son, then an heir of God through Christ.

Nevertheless what saith the scripture? Cast out the bondwoman and her son: for the son of the bondwoman shall not be heir with the son of the freewoman.