Block Quote where the 4 women mentioned in the Matthew genealogy are provided with reasons. The Expositors Greek Testament

Matthew 1:3

And Judas begat Phares and Zara of Thamar; and Phares begat Esrom; and Esrom begat Aram;

Matthew 1:3. τὸν φαρὲς καὶ τὸν Ζαρὰ: Zerah added to Perez the continuator of the line, to suggest that it was by a special providence that the latter was first born (Genesis 38:27-30). The evangelist is on the outlook for the unusual or preternatural in history as prelude to the crowning marvel of the virgin birth (Gradus futurus ad credendum partum e virgine. Grot.).—ἐκ τῆς Θάμαρ. Mention of the mother wholly unnecessary and unusual from a genealogical point of view, and in this case one would say, primâ facie, impolitic, reminding of a hardly readable story (Genesis 38:13-26). It is the first of four references to mothers in the ancestry of Jesus, concerning whom one might have expected the genealogy to observe discreet silence: Tamar, Rahab, Ruth, Bathsheba; three of them sinful women, and one, Ruth, a foreigner. Why are they mentioned? By way of defence against sinister misconstruction of the birth of Jesus? So Wetstein: Ut tacitæ Judaeorum objectioni occurreretur. Doubtless there is a mental reference to that birth under some aspect, but it is not likely that the evangelist would condescend to apologise before the bar of unbelief, even though he might find means of doing so in the Jewish habit of glorying over the misdeeds of ancestors (Wetstein). Much more probable is the opinion of the Fathers, who found in these names a foreshadowing of the gracious character of the Gospel of Jesus, as it were the Gospel in the genealogy. Schanz follows the Fathers, except that he thinks they have over-emphasised the sinful element. He finds in the mention of the four women a hint of God’s grace in Christ to the sinful and miserable: Rahab and Bathsheba representing the one, Tamar and Ruth the other. This view commends itself to many interpreters both Catholic and Protestant. Others prefer to bring the four cases under the category of the extraordinary exemplified by the case of Perez and Zerah. These women all became mothers in the line of Christ’s ancestry by special providence (Weiss-Meyer). Doubtless this is at least part of the moral. Nicholson (New Comm.) thinks that the introduction of Tamar and Ruth is sufficiently explained by Ruth 4:11-12, viewed as Messianic; of Rahab by her connection with the earlier Jesus (Joshua), and of Bathsheba because she was the mother of a second line culminating in Christ, as Ruth of a first culminating in David. 

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