Galatians 4:23
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.
Hebrews 11:11
By [having] faith, Sarah herself received the ability to have children, even though she was [sterile, being] past the age of child-bearing, since she considered God faithful to His promise. [Note: Some translations consider "Abraham" to be the subject of this highly controversial verse. See Lightfoot, pages 222-225].
Romans 9:7-8
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."
Romans 4:18-21
Abraham hoped for what he believed [God had promised], even when there seemed little hope [of it happening], so this made him forefather of many nations, according to the words spoken to him [Gen. 15:5], "Your descendants will be many."
Romans 10:8
But what does the Scripture say [about being made right with God]? [Deut. 30:14 says], "The message of God is near you [i.e., it is not difficult to comprehend]. It is on your lips and in your heart." This is the message of faith [in Christ] which we [apostles] preach.
Galatians 4:28-29
Now, brothers, you [Christians] are children of the promise [made to Abraham. See 3:29] just like Isaac, [Abraham's son] was.