Dear Suzanne:

This story is in Genesis 6:1-8 and is cited as the reason for Noah's flood:

When men began to multiply on the face of the ground, and daughters were born to them, the sons of God saw that the daughters of men were fair; and they took to wife such of them as they chose. Then the LORD said, "My spirit shall not abide in man for ever, for he is flesh, but his days shall be a hundred and twenty years." The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of men, and they bore children to them. These were the mighty men that were of old, the men of renown.

The LORD saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually. And the LORD was sorry that he had made man on the earth, and it grieved him to his heart. So the LORD said, "I will blot out man whom I have created from the face of the ground, man and beast and creeping things and birds of the air, for I am sorry that I have made them. But Noah found favour in the eyes of the Lord.

This is story of the spread of wickedness of Man that was so great that it prompted God to regret the creation of Man and thus He caused a flood to destroy all but Noah and his family.

The wickedness is represented by the "sons of God" marrying the "daughters of men".

There are many people, especially some Protestants, who assert that Fallen Angels mated with human women to produce the Giant people known as Nephlims. This is not possible, however.

Angels (of God or of Satan) are pure spirits. They have no biological functions in which to procreate. For angels to impregnant a woman is utterly impossible.

When the Holy Spirit impregnated Mary that was a miracle performed by God. Only God can perform miracles, angels cannot.

The Navarre Bible Commentary gives a good explanation of these passages. To quote and paraphrase that commentary...

...There were some ancient Jewish tradition, and even some early Christian writers, who interpreted "sons of God" and fallen angels. This stems from an Yahwistic myth in vogue in ancient times that giants were the offspring of women who mated with being of a higher order. As a result of this sin God restricted the life-span of Man to 120 years.

But, since it is impossible for a spirit to impregnant a woman, there has to be some other explanation.

The explanation given by St. Augustine, St. Cyril of Alexandra, and other Church Fathers is that the "sons of God" means good men, the descendants of Seth, who indiscriminately took as wives the descendants of Cain ("daughters of men"). The line of Cain was cursed because Cain killed his brother Abel. As such, the descendants of Seth had no business marrying women from the line of Cain. This mating caused corruption and is a biblical example of why we should not marry outside of our faith.

It is Mankind's moral decline on account of pride and abuses of marriage that prepares the way for the Flood.

Such a society condition is very much like today.

What we know for sure is that neither good or evil spirits can impregnate humans since they have no biology to accomplish such a feat and they cannot perform miracles.

God Bless,
Bro. Ignatius Mary