This isn't a biblical teaching, just a cult teaching.
Genesis 6:4 The Nephilim were in the earth in those days, and also after that, when God's sons came to men's daughters. They bore children to them: the same were the mighty men who were of old, men of renown.
Ge 6:4
1 sons of God
Some hold that these "sons of God" were the "angels which kept not
their first estate" Jude 1:6. It is asserted that the title is in the O.T.
exclusively used of angels. But this is an error Isa 43:6. Angels are
spoken of in a sexless way. No female angels are mentioned in
Scripture, and we are expressly told that marriage is unknown
among angels. Mt 22:30.
The uniform Hebrew and Christian interpretation has been that verse
Ge 6:2 marks the breaking down of the separation between the
godly line of Seth and the godless line of Cain, and so the failure of
the testimony to Jehovah committed to the line of Seth Ge 4:26. For
apostasy there is no remedy but judgment Isa 1:2-7,24-25; Heb
6:4-8; 10:26-31. Noah, "a preacher of righteousness," is given 120
years, but he won no convert, and the judgment predicted by his
great-grandfather fell Jude 1:14-15; Ge 7:11. ~Scofield