John Calvin
Gen 6
1. And it came to pass, when men began to multiply. Moses, having enumerated in order, ten patriarchs, with whom the worship of God remained pure, now relates, that their families also were corrupted. But this narration must be traced to an earlier period than the five hundredth year of Noah. For, in order to make a transition to the history of the deluge, he prefaces it by declaring the whole world to have been so corrupt, that scarcely anything was left to God, out of the widely spread defection. That this may be the more apparent, the principle is to be kept in memory, that the world was then as if divided into two parts; because
the family of Seth cherished the pure and lawful worship of Good, from which
the rest had fallen. Now, although all mankind had been formed for the worship of God, and therefore sincere religion ought everywhere to have reigned; yet since the greater part had prostituted itself, either to an entire contempt of God, or to depraved superstitions; it was fitting that the small portion which God had adopted, by special privilege, to himself, should remain separate from others. It was, therefore, base ingratitude in
the posterity of Seth, to mingle themselves with the children of Cain, and with other profane races; because they voluntarily deprived themselves of the inestimable grace of God. For it was an intolerable profanation, to pervert, and to confound, the order appointed by God. It seems at first sight frivolous, that the sons of God should be so severely condemned, for having chosen for themselves beautiful wives from the daughters of men. But we must know first, that it is not a light crime to violate a distinction established by the Lord; secondly, that for the worshippers of God to be separated from profane nations, was a sacred appointment which ought reverently to have been observed, in order that a Church of God might exist upon earth;
http://www.ccel.org/c/calvin/comment3/comm_vol01/htm/xii.htm
