The following is an excerpt from Doug Hamp’s blog, DouglasHamp.com:
The notion that Genesis 6 ‘sons of God‘ is a reference to ‘the sons of Seth’ is surprisingly popular despite the fact that the Bible is replete with evidence that the sons of God in Genesis 6:1-4 were fallen angels (demons) and despite the fact that all of the ancient Jewish and Ante-Nicene Christian commentators believed the “sons of God” to be referring to demons (fallen angels).
Augustine of Hippo
The first, as far as we can see, to definitively deny the sons of God as being angels was Augustine of Hippo of the fifth century, approximately seventy five years after the drafting of the Nicene Creed. Augustine did much to spiritualize the history of the Bible and twist a simple straightforward reading of the Bible. His method of Bible interpretation made a profound impact and his legacy remains even to this day.