The interpretation of the sons of God in Gen. 6.1-4 as angels dominates interpretation in pre-Christian and early Christian times. The Septuagint and 1 Enoch 6ff interpret it this way – possibly in the third century BC – as do many subsequent works and authors, including the pre-Christian Jewish works of Jubilees, the Damascus Document, and Philo, and the post-Christian Josephus, 2 Baruch, Justin Martyr, Irenaeus, etc.
It is often contended that the first Jewish witnesses to a non-angelic interpretation are in Aquila’s and Symmachus’s translations of the passage into Greek, in the second century AD, or perhaps Philo’s interpretation in the first century AD. For example, here’s Robert C. Newman (Grace Theological Journal 5.1 (1984): 13-36):
The earliest extant examples of the nonsupernatural interpretations of Gen 6:2, 4 come from the 1st century AD and thus are later than the earliest specimens of the supernatural interpretation. Since all come centuries after Genesis was written, it is not possible to be sure which is the oldest.
The non-angelic (and non-divine) interpretation is often also declared to be a result of an anti-angelic bias following the fall of Jerusalem and the subsequent reaction against various Jewish sects with their angelic speculations. For this reason, the non-angelic interpretation is considered a late novation. Thom Stark, in his recent book The Human Faces of God (p. 77), makes mention only of post-70 AD examples of the interpretation of Gen. 6.1-4 as involving human princes.
As Christians continued to regard 1 Enoch as scripture until at least the third century AD, the angelic interpretation of Gen. 6.1-4 continued long after it was rejected in Rabbinic Judaism. Although Julius Africanus mentions the non-angelic interpretation in the early third century AD, it is Augustine who defends it in detail, in Book 15 of the City of God, in the early fifth century AD.
But often overlooked in these discussions is Sir. 16.7 – which appears in a section of Sirach (ca. 180 BC) dealing with God’s “historical” punishment of sinners (Sir. 16.5-10). The New Revised Standard Version renders it:
He did not forgive the ancient giants
who revolted in their might.
The words “ancient giants” (γιγάντων τῶν ἀρχαίων) were only a Greek translation of Jesus ben Sirach’s original Hebrew, carried out by his grandson. The Hebrew (of both mss A and B) reads נסיכי קדם (“princes of old”).
What did Sirach mean by “princes of old” who revolted in their might? While “princes” could possibly refer to angelic princes (cf eg Dan 10.13), all the other examples in this list of ancient sinners are human: Korah and party (16.6), Sodom and Gomorrah (16.8), the Canaanites (16.9), and the exodus generation (16.10). So we might reasonably expect the same in 16.7. In addition, the words “in their might” (בגבורתם) clearly allude to the Gibborim of Gen. 6.4b, thus identifying the princes of old with those Gibborim or “men of renown” – again, mortal humans. In addition, the reference to “revolt” appears to identify the figures in Sir. 16.7 with “the sons of God” in Gen. 6.2, 4 – as it is the sons of God who do the revolting in the biblical passage. Further, the term “sons of God” is most usually a term for human kings or princes, which may explain Jesus ben Sirach’s word choice here.
In conclusion, there is a good case for dating the non-angelic interpretation of Gen. 6.1-4 as early as the writing of Sirach (ca. 180 BC) – and for concluding that, far from a post-70 Jewish reactionary interpretation, the non-angelic interpretation could be as old as the angelic interpretations found in LXX and 1 Enoch.
Update (23 Feb 2012): Jim Davila comments on this issue, and opines that Sir. 16.7 is limited to the giants, and that the “revolt” is also that of the giants (cf. Gen. 6.11) and not the sons of God. We both agree, I think, that Gen. 6.4 distinguishes the Nephilim=offspring=Gibborim=men of renown from the sons of God. But I’d add that in Sir. 16.7’s interpretation of Gen 6:4, the princes/giants are royal heroes, sons of similar royal heroes (themselves “sons of God/s”).
Update 2 (19 March 2012): I just discovered that one of the world’s leading biblical gigantologists, Matthew Goff, has a fairly recent article on the giants of Sirach 16.7 (in JBL 129.4). He has a slightly different take on the nature of the allusion to Genesis 6.1-4 and, as always, some valuable insights.
Update 3 (4 January 2016): Jim Davila notes that Matthew Goff’s paper has been recently uploaded to Academia. Goff makes a case that the princes/giants of Sirach 16.7 are not the Nephilim/Gibborim/men of renown of Genesis 6.1-4, but “aboriginal Canaanite rulers with language that evokes Genesis 6″, such as those represented in Num 13. Maybe. But against this interpretation, the list of punished sinners in Sirach 16.7-10 does seem to proceed in more-or-less straightforward chronological fashion: Nephilim/Gibborim/princes of Gen 6:1-4, punished by the Flood (16.7); the people of Sodom, punished in destruction of Sodom (16.8); the Canaanites as great sinners, punished by the invading Israelites (16.9); the desert generation of Israelites, punished by God’s confinement of them to the desert (16.10). I am not finally convinced by Goff’s other arguments in the paper, but the proposal is well worth consideration.