But, in addition to the point upon which we have been dwelling, there was yet another question to be settled ere Abraham could reach the most elevated point of his course as a man of faith. The bondwoman and her son were yet in the house. He must put forth these from _his house_ as he had put forth the evil from _his heart_. The house and the heart must be cleared out. In chapter xxi. we find matters brought to a crisis with regard to the bondwoman and her son, concerning whom we have heard comparatively nothing until now. The element of bondage had heretofore lain dormant in Abraham's house because not roused into action, by anything of an opposite nature and tendency. But, in the birth of Isaac--the son of the free woman--the child of promise--we see a new element introduced. The spirit of liberty and the spirit of bondage are thus brought into contact, and the struggle must issue in the expulsion of either one or the other. They cannot move on in harmony, for "how can two walk together except they be agreed." Now we are invited by the Apostle, in his epistle to the Galatians, to behold in these two children, "the two covenants," the one gendering to _bondage_, the other to _liberty_; and further, to behold in them samples of the fleshly and spiritual seed of Abraham, the former, "born after the flesh," the latter, "born after the Spirit." Nor can anything be more marked than the line of demarcation between, not only the two covenants, but the two seeds. They are totally distinct the one from the other, and can never, by any operation, be brought to coalesce. Abraham was made to feel, and that painfully, this fact. "Cast out this bondwoman and her son; for the son of this bondwoman shall not be heir with my son, even with Isaac" (Chap. xxi. 10). Here the natural result shows itself. The two elements could not mingle. As well might the north and the south winds be expected to blow in all their strength without exciting a convulsion in the elements.