The phrase "will be named" means those born through [Isaac](rc://en/tw/dict/bible/names/isaac) are the ones God considers to be the descendants that he promised [Abraham](rc://en/tw/dict/bible/names/abraham). This can be stated in active form. AT: "[Isaac](rc://en/tw/dict/bible/names/isaac) is the one who will be the forefather of the descendants I promised to give you" (See: [[rc://en/ta/man/translate/figs-activepassive]])