That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose.
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George Leo Haydock
The sons of God. The descendants of Seth and Enos are here called Sons of God, from their religion and piety: whereas the ungodly race of Cain, who by their carnal affections lay grovelling upon the earth, are called the children of men. The unhappy consequence of the former marrying with the latter, ought to be a warning to Christians to be very circumspect in their marriages; and not to suffer themselves to be determined in choice by their carnal passion, to the prejudice of virtue or religion. (Challoner)
See St. Chrysostom, hom. 22 Some copies of the Septuagint having the angels of God, induced some of the ancients to suppose, that these spiritual beings (to whom, by another mistake, they attributed a sort of aerial bodies) had commerce with women, as the pagans derived their heroes from a mortal and a god. But this notion, which is borrowed from the book of Henoch, is quite exploded. (Calmet)
The distinction of the true Church from the synagogue of satan, here established, has been ever since retained, as heretics are still distinguished from Catholics. (Worthington) (St. Augustine)