4:21-27 The difference between believers who rested in Christ only, and those who trusted in the law, is explained by the histories of Isaac and Ishmael. These things are an allegory, wherein, beside the literal and historical sense of the words, the Spirit of God points out something further. Hagar and Sarah were apt emblems of the two different dispensations of the covenant. The heavenly Jerusalem, the true church from above, represented by Sarah, is in a state of freedom, and is the mother of all believers, who are born of the Holy Spirit. They were by regeneration and true faith, made a part of the true seed of Abraham, according to the promise made to him.
26. This verse stands instead of the sentence which we should expect, to correspond to Ga 4:24, "One from Mount Sinai," namely, the other covenant from the heavenly mount above, which is (answers in the allegory to) Sarah.
Jerusalem … above—(Heb 12:22), "the heavenly Jerusalem." "New Jerusalem, which cometh down out of heaven from my God" (Re 3:12; 21:2). Here "the Messianic theocracy, which before Christ's second appearing is the Church, and after it, Christ's kingdom of glory" [Meyer].
free—as Sarah was; opposed to "she is in bondage" (Ga 4:25).
all—omitted in many of the oldest manuscripts, though supported by some. "Mother of us," namely, believers who are already members of the invisible Church, the heavenly Jerusalem, hereafter to be manifested (Heb 12:22).