Peter does not specify the sin. Jude says they "kept not their first
estate, but left their own habitation." This, taken in connection
with Deut. 32:8, which seems to indicate that certain territories
or boundaries were appointed unto the angels, and Gen. 6:1-4,
which speaks of the "sons of God" (which some suppose to refer to
angels, which, however, is questionable), might seem to imply that
the sin of the angels consisted in leaving their own abode and coming
down to cohabit with the "daughters of men." Thus their sin would
be that of lust. To some expositors the context in Jude would
seem to warrant such a conclusion, inasmuch as reference is made
to the sins of Sodom and Gomorrah. But this can hardly be true,
for a close study of the text in Genesis 6 shows that by "the sons
of God" are meant the Sethites. This would seem to be the true
interpretation; if so, then the sin recorded in Genesis 6 would
be (1) natural and not monstrous; (2) Scriptural, and not mythical
(cf. Num. 25; Judges 3:6; Rev. 2:14, 20-22 contains sins of a similar
description); (3) accords with the designations subsequently given
to the followers of God (Luke 3:38; Rom. 8:14; Gal. 3:26); (4) has
a historical basis in the fact that Seth was regarded by his mother
as a (the) son of from God, (5) in the circumstance that already
the Sethites had begun to call themselves by the name of Jehovah
(Gen. 4:26); (6), finally, it is sufficient as a hypothesis, and
is therefore entitled to the preference (after Lange).
There are still others who say that the sin of the angels was pride
and disobedience. It seems quite certain that these were the sins
that caused Satan's downfall (Ezek. 28). If this be the true view
then we are to understand the words, "estate" or "principality" as
indicating that instead of being satisfied with the dignity once
for all assigned to them under the Son of God, they aspired higher.