Now [specific] promises were made to Abraham and to his seed [i.e., descendants]. [God] did not say "seeds" [plural] as though He were referring to many persons but "seed" [singular] showing that He meant one [descendant]; [Gen. 13:15 says] "And to your seed," referring to Christ.

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].

Now this woman, Hagar, is similar to Mount Sinai, in Arabia, and represents the present [earthly] Jerusalem [i.e., Israel], consisting of children in bondage [to the law of Moses].