4 The Nephilim were on the earth in those days—and also afterward—when the sons of God went to the daughters of humans and had children by them. They were the heroes of old, men of renown. Genesis 6:4
While this passage is a mystery to many, I will not dwell on the details as smarter minds than mine have studied it and been stumped. R.C. Sproul Jr. thus summarized it, and it makes sense:
When we consider the context of this text we can better understand what Moses is explaining. In previous chapters we are given a glimpse of two competing lines, the godly line of Seth and the wicked line of Cain. Having established the antithesis in the garden, after affirming that there would be a constant struggle between the seed of the woman and the seed of the serpent we are given snapshot pictures of each of these armies. We see Seth’s line about the business of exercising dominion, in submission to the Lord. We see Cain’s line dishonoring the law of God and making names for themselves. But the future is not mere co-existence between the two lines. The drama builds toward the great crisis of Noah’s flood right here in chapter 6. The great change, what creates the great downward spiral of humanity on the earth is that the two lines come together as one. That is, the godly line of Seth, the sons of God, seeing how attractive are the daughters of men, the wicked line of Cain, decide to take them as wives. The end result, however, isn’t mere dilution. It’s not that the now joined line becomes morally lukewarm, but that evil spreads, grows, deepens. This shouldn’t surprise as for as Chuck Swindoll reminds us, if you drop a white glove in the mud, the mud doesn’t get all glovey.
So, in summary, the passage as I understand it, is akin to being unequally yoked as stated in the New Testament. They were marrying of a different faith altogether. The evil in the world continued to spread as a result of the union between Seth’s lineage and his brother Cain’s lineage.
The bad news is, it only gets worse from here.
The good news, however, is that God will have a plan for it all. He always does.