Abraham's faith did not become weak [even though] he was about one hundred years old and considered his body almost dead and Sarah's womb incapable of conceiving a child.

For these are the words of the promise [Gen. 18:10], "I [i.e., God] will come at the appointed time and Sarah will have a son."

For it is written in the law about Abraham's two sons [Gen. 16]; one [was] by his slave woman [Hagar] and the other by the free woman, [his wife, Sarah].

This set of circumstances contains an illustration; for these two women [Hagar and Sarah] represent the two Agreements [between God and mankind]. The one [Agreement], given at Mount Sinai, represents Hagar, whose children became [the Israelites], enslaved under bondage [to the requirements of the law of Moses].

Therefore, brothers, we [Christians] are not the [spiritual] children of the slave woman [Hagar] but of the free woman [Sarah].

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].

Sarah obeyed her husband Abraham, even referring to him [in respectful terms such] as "sir." You [wives] are her [spiritual] descendants if you also do what is right and are not fearful [that things in life will turn out badly].