malachi 2:15 And did not he make one? Yet had he the residue of the spirit. And wherefore one? That he might seek a godly seed. Therefore take heed to your spirit, and let none deal treacherously against the wife of his youth. - Free Bible Online

      15. MAURER and HENGSTENBERG explain the verse thus: The Jews had defended their conduct by the precedent of Abraham, who had taken Hagar to the injury of Sarah, his lawful wife; to this Malachi says now, "No one (ever) did so in whom there was a residue of intelligence (discriminating between good and evil); and what did the one (Abraham, to whom you appeal for support) do, seeking a godly seed?" His object (namely, not to gratify passion, but to obtain the seed promised by God) makes the case wholly inapplicable to defend your position. MOORE (from FAIRBAIRN) better explains, in accordance with Mal 2:10, "Did not He make (us Israelites) one? Yet He had the residue of the Spirit (that is, His isolating us from other nations was not because there was no residue of the Spirit left for the rest of the world). And wherefore (that is, why then did He thus isolate us as) the one (people; the Hebrew is 'the one')? In order that He might seek a godly seed"; that is, that He might have "a seed of God," a nation the repository of the covenant, and the stock of the Messiah, and the witness for the one God amidst the surrounding polytheisms. Marriage with foreign women, and repudiation of the wives wedded in the Jewish covenant, utterly set aside this divine purpose. CALVIN thinks "the one" to refer to the conjugal one body formed by the original pair (Ge 2:24). God might have joined many wives as one with the one husband, for He had no lack of spiritual being to impart to others besides Eve; the design of the restriction was to secure a pious offspring: but compare Note, see on Mal 2:10. One object of the marriage relation is to raise a seed for God and for eternity.

JFB.