Genesis 6 indicates that the "sons of God" (B'nai Elohim) took wives of the "daughters of men," which gave birth to the "Nephilim." What on earth was going on?
The B'nai Elohim is a term that refers to angels. It occurs four times in the Old Testament and is rendered "Angels of God" in the ancient Septuagin translation. The intrusion of certain angels into the human family resulted in unnatural offspring termed Nephilim, which derives from the Hebrew naphal (to fall), or the Fallen Ones. (The Greek Septuagint renders this term gigantes, which actually means "earth-born." This is often misunderstood to mean "giants"--which they also happen to have been, incidentally.)
"Line of Seth?"
The early church viewed the B'nai Elohim as angels up through the late fourth century: Justin, Athenagoras, Cyprian, Eusebius, et al. (also Josephus, Philo, Judeaus, and the Apochrypha regard this view).
Celsus and Julian the Apostate exploited the older common belief to attack Christianity. Cyril of Alexandria, in his reply, repudiated the orthodox position. Julius Africanus (a contemporary of Origen) introduced the theory that the "sons of God" simply referred to the genealogical line of Seth, which was committed to preserving the true worship of God.
Seemingly more appealing, the "Sethite theory" prevailed into the Medieval Church, and many still hold this view.
This view, however, has several serious problems. There is no indication that the Sethites were distinguished for piety; they were not exempted from the charge of general wickedness which brought on the flood. In fact, Seth's son Enosh was the one who introduced apostasy to that world. This is masked by a mis-translation of Genesis 4:25, which should read:
"...then men began to profane the name of the Lord." Furthermore, when the faithful marry the unfaithful, they do not give birth to unnatural offspring! And the "daughters of men" were not differentiated with regard to the Flood. All were lost.
(Incidentally, the Nephilim didn't completely end with the flood. Genesis 6:4 mentions, "...and also after that..." We find the sons of Anak, the Anakim, later in the Old Testament.)
The Reason for the Flood
It was the infusion of these strange beings into the human predicament that brought on the Flood of Noah. The Flood was preceded by four generations of prophets/preachers warning of the coming judgment: Enoch, Methuselah, Lamech, and Noah.
It seems that this was part of Satan's stratagem to corrupt the line of Adam to prevent the fulfillment of the Messianic redemption. Noah was apparently unique in that his genealogy was still uncorrupted.
The strange events which led to the flood are also alluded to in ancient mythologies. The legends of the Greek "titans"--partly terrestrial, partly celestial--embrace these same memories.
The Angels that Sinned
There is a great deal revealed in the Bible about angels. They can appear in human form, they spoke as men, took men by the hand, even ate men's food, are capable of direct physical combat, some are the principal forces behind the world powers. They don't marry (in Heaven), but apparently are (or were) capable of much mischief.
The strange events of Genesis Chapter 6 are also referred to in the New Testament. Peter refers to events preceding the flood of Noah:
"For if God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment..." 2 Peter 2:4
(Peter uses the term tartarus, here translated as hell. This was a Greek term for "the dark abode of woe, the pit of darkness in the unseen world." Homer's Iliad portrays tartarus "as far below hades as the earth is below Heaven...")
Also, in Jude, it mentions them:
"And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day. Even as Sodom and Gomorrah, and the cities about them in like manner, giving themselves over to fornication, and going after strange flesh, are set forth for an example, suffering the vengeance of eternal fire." Jude 6,7.
Scripture warns against mepling with the spirit world. The punishment which overtook the angels that sinned was to emphasize the serious nature of apostasy: beings of a higher order than ours have been hurled down into a dark place of confinement where they have remained for thousands of years.
God has not changed His attitude toward them; time has not mitigated the seriousness of their sin. False teachers are pre-written into condemnation.
The "Sons of God" Return?
There are many who believe that the recent "alien" involvements are also demonic and are just another precursor to the end-time. Some also believe that the Coming World Leader may boast of an "alien connection." It would be consistent from what else we can infer from Scripture.