I checked out Deut 14:1 and while it shares 2 roots in common, the phrase differs in that it has an additional word in the middle. The two phrases are not comparable. The comparable phraseologies all fall in Job, which most scholars suggest is actually a work that predates Moses. As the two oldest writings in the Bible (the Tanakh and Job), they would share usage.
Job is a different genre (or more appropriately, mixture of genres), from a different period, written by a possibly different author. Even if I conceed the predating of Job's experience, the time of writing predating Moses is on tenuous grounds. I have not come across any allusions of Moses to Job at all. Do you have one in mind apart from this case?
Your blanket dismissal of the similarity of the phrase "sons of the Lord your God" to "sons of God" is rather unconvincing. By then the word "son" carried strong connotations of heir, seed, and of the same family. Look at the continual reference to "sons of Israel" in the first 5 books. Israel had passed away years ago, yet they were all still refered to as "sons of Israel." The phrase, "sons of God" would carry the same sort of relationship connotation.
So then the question becomes, what is the case for Moses' use of the phrase to mean "sons of Seth", when it doesn't even make logical sense to declare that the only God-followers were descended from him? Why not other sons and daughters of Adam? And again, what makes the daughters of men (actually Adam) equated with Cain? Both of these contestations require a logical leap based on conjecture, while to discount my view, you maintain that despite the ancient usage of the phrase in Job, which Moses would have been aware of, Moses actually meant something else.
I simply maintain that Moses means the same relationship with the phrase "sons of God" as he does when he uses the similar phrase, "sons of Israel." I deny that Job's usage has anything other than a tangental impact on how we should interpret Moses. Once again, James and Paul used different definitions of righteousness, we don't force one meaning into the other, we reach each text on its own grounds. I will deal with the concept of two people of God below. I am willing to say that it may not have started with Seth, but Seth is simply set off in contrast to Cain. We could instead say "Adam" as the first son of God, but that confuses where Cain fit into the story (as he clearly does not fit in as a "son of God").
Lastly, the angel view does appear to be corroborated with 1 Peter and Jude. How would you explain those NT references with the Sethite view? It renders them incomprehensible.
I'm not too sure what you mean by 1 Peter corroborating the angel view. Are you saying that the "spirits in prison" (1 Peter 3:18-22) are angels? You would have to do some exegesis here to support that. I would suggest that the "spirits in prison" is built off of the previous verse, which refers to us having put to death the flesh and become alive in the spirit. In contrast, the gospel was preached to those who had not done so, the rebellious people of the time who had not put to death the flesh and thus had spirits that were in prison.
Jude doesn't pose any threat to this specific interpretation of the phrase referring to the sons of Seth, all it says is that there were angels that left their positions. We don't even necessarily have to understand them as committing sexual immorality, but only that the cities did so, just like Sodom and Gomorrah. (Jude 5-7)
Certainly not "incomprehensible" by any stretch of the imagination!
Finally, in a previous comment, Shane writes:
What it really comes down to is that this dichotomy requires that all humans fall into one group or the other. It doesn't hold water though because of one verse: Genesis 5:4. Adam had other sons and daughters. Are they of Seth or Cain? Neither. So how can we say that all these lines were mandated to be separate? The first time Scripture clearly sets people apart like you want to do is not until Abraham.
Why does it matter that Adam has other sons and daughters? The focus of the text is clearly upon two lines of people, Seth and Cain, fulfilling the eternal battle between the seed of the woman and the seed of the serpent, which is where I would say that mankind is first seperated into two classes. Birth was not the determining factor, but the sovereign act of God was. For some He decreed that Satan and death would reign in their lives, for others that His righteousness through His Son would.
We see this distinction immediately with Cain's opposition of Abel. Abel pleases God, Cain does not. Cain murders Abel. Cain is then cursed from the ground. From Cain comes Enoch, then through him down we get Lamech, who institutes polygamy and in an act of sinful pride, declares that he doesn't need God's protection! God had protected Cain sevenfold, but Lamech declares that his revenge will be seventy-sevenfold. (Genesis 4)
Genesis 5 then begins the focus on the other line. God, being a God who doesn't name people randomly, brings about a marked similarity between the names of the two groups of people. Enoch from the line of Seth is righteous and walks with God. Lamech shows up and instead of declaring that he doesn't need God, he fathers Noah and lives 777 years (a possible sign of blessing?). Noah was righteous.
Now we come to Genesis 6. One of the lines disappears and the other is continued in Noah. One is wiped from the map and the other is saved through water.
So what do we see? God's line of people include (but is not limited to) the line of Noah. The line of descendants from Cain is clearly marked off in contrast to the line of descendants from Seth. Yet, even of the line of Seth, only a few are saved, the rest perish because of their wickedness and evil thoughts as just punishment for their sins. What were some of those sins? Intermarriage with the line of Cain, the daughters of man. The sons of God, deemed "family" with God's people, intermarried with the seed of the serpent, the daughters of man.
That's all I have to say on this matter. Shane, if you like, you may have the last word.