For this reason, the promise [i.e., of being made right with God] comes through faith [in God], according to His unearned favor, so that it may be [given] with certainty to all of Abraham's descendants. The promise is given not only to those [who live] under the law [i.e., the Jews], but also to [all] those [who live] by faith, like Abraham. Abraham is the [spiritual] father of all of us [believers],

Neither are all [Jewish] people children [of God], [simply] because they are Abraham's descendants. But [Gen. 21:12 says], "It is through Isaac that you [i.e., Abraham] will have descendants."

This means that it is not the physical descendants [of Abraham] who are God's children, but it is the children [born] according to [God's] promise who are [Abraham's true] descendants.