This illegal marriage between fallen angels and the human race produced Nephilim. The "mighty men which were of old, men of renown" in 6:4 were a product of the fallen spirits mingled with the human race (cf. Num. 13:33). Although the King James Version uses the word "giants" in Genesis 6:4 and Numbers 13:33, the Hebrew should be rendered "Nephilim." It is translated this way in the American Standard Version. The word Nephilim means "the fallen ones." G. H. Pember in his book Earth's Earliest Ages says that it probably refers to the fallen angels. However, the Nephilim were not purely angels, but were the product of the mixture of the fallen angels with the daughters of men. They were the product of the mingling of the human race, that is, human blood, with angelic spirits.
According to the Bible, the Nephilim were "men of a great stature" (Num. 13:32). If you were to look at them, they would threaten you, and you would be frightened. That is exactly what happened to ten of the twelve spies sent by Moses to explore the land of Canaan. They saw the Nephilim and were terrified (Num. 13:33). They told the Israelites, "We should not go into the land, for we saw the Nephilim there." The Nephilim which they saw should be again the descendants of the fallen angels that had mingled with the daughters of men. The Nephilim were giants, capable men, men of renown.
God sent the flood to exterminate Noah's generation because the blood of that generation had become impure. Likewise, God slaughtered all the Canaanites because the Nephilim again were among them, and the blood of that race again was no longer pure. God could not tolerate that situation. For the sake of the fulfillment of His purpose, He could not allow such a human race to exist.
We all must learn one lesson from this: fornication is a serious thing. In the entire Bible, especially in the teaching of the Lord Jesus (Matt. 19:9), fornication is regarded as a serious matter. When you read the Epistles in the New Testament, you can see that the apostles were very strict, not tolerating fornication in the church life (1 Cor. 6:9-10, 18; Eph. 5:5). Why were they so strict? If you read the Bible, you will discover that fornication is related to idolatry (Rev. 2:14, 20). Idol worship encourages fornication. In Buddhism, all the people who worship idols are careless about fornication. Their idol worship ushers them into fornication. In China, in front of the idol temples where people worshipped idols, the opera was performed quite often with the stories of fornication. If you read the book of Numbers, you will see that when the Israelites worshipped idols, they committed fornication (25:1-2). The worship of idols brings in fornication, and fornication ushers in idolatry. But this is not true in the genuine worship of God. The true worship of God keeps us pure with respect to marriage, maintaining the governing principle ordained by God. Several years ago a group of young people living together in San Francisco became involved in fornication. That fornication eventually brought them into witchcraft. Fornication brings people into contact with demons. Why? Because demons, fallen spirits, are searching for human bodies. If you are a person who exercises a proper intellect and reason, you will not allow an evil spirit to come into you. However, when people become addicted to fornication, they do not care about their intellect. They lose their reason, and the door is opened for demons to enter in and take full possession of them. None of the people who commit fornication care for their intellect or reason. They are not sound or normal. If you are a sound, normal person, you will care for your intellect and reason. You will not do anything which breaks God's governing principle. It is a dreadful thing to fall into fornication.
According to the record of the Bible, at the time of the third fall, men became flesh. They were full of lusts. Because men were in such a condition, the door was opened for the angels, the fallen ones, to enter in. At this point, man not only had the Devil in his evil nature, but some men had become possessed by the fallen angels who used their bodies to form illegal marriages and produce a mixture. That was the most evil thing in the eyes of God, and He could not tolerate it.