John Cassian

As long then as there continued that separation of the lines between them, the seed of Seth, as it sprang from an excellent root, was by reason of its sanctity termed "angels of God," or as some copies have it "sons of God;" and on the contrary the others by reason of their own and their fathers' wickedness and their earthly deeds were termed "children of men." Though then there was up to this time that holy and salutary separation between them, yet after this the sons of Seth who were the sons of God saw the daughters of those who were born of the line of Cain, and inflamed with the desire for their beauty took to themselves from them wives who taught their husbands the wickedness of their fathers, and at once led them astray from their innate holiness and the single-mindedness of their forefathers...From these sons of Seth then and daughters of Cain, as we have said, there were I born still worse children who became mighty hunters, violent and most fierce men who were termed giants by reason of the size of their bodies and their cruelty and wickedness. For these first began to harass their neighbours and to practise pillaging among men, getting their living rather by rapine than by being contented with the sweat and labour of toil, and their wickedness increased to such a pitch that the world could only be purified by the flood and deluge.