If the first portion [of dough offered as the first fruits] is holy, so is the whole batch; and if the root (Abraham, the patriarchs) is holy, so are the branches (the Israelites).

and they are not all the children of Abraham because they are his descendants [by blood], but [the promise was]: “Your descendants will be named through Isaac” [though Abraham had other sons].

That is, it is not the children of the body [Abraham’s natural descendants] who are God’s children, but it is the children of the promise who are counted as [Abraham’s true] descendants.