You people are descendants of these prophets and [recipients] of the Agreement God made with your forefathers when He said to Abraham [Gen. 12:3], ‘And through your seed [i.e., your descendant Jesus] all the people of the earth will receive the blessings [of God].’
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.”
And if you belong to Christ, then you are [truly] Abraham’s seed [i.e., his spiritual descendants] and thereby [you will] inherit what was promised to him [i.e., God’s blessings].
Now the son [Ishmael], born to the slave woman, was born simply of their natural relationship; but the son [Isaac], born to the free woman, was born in fulfillment of a [divine] promise.