The Bible mentions that they were disobedient angels that left their place in heaven during the time of Noah, came down to the earth, and took on fleshly bodies:
"The angels that did not keep their original position but forsook their own proper dwelling place..." (Jude 6)
Their reason for doing this is explained in the following scripture:
“The sons of the true God began to notice the daughters of men, that they were good-looking; and they went taking wives for themselves, namely, all whom they chose.” (Genesis 6:2)
Supporting the conclusion that these were ANGELIC "sons of God" is the apostle Peter’s references to “the spirits in prison, who had once been disobedient when the patience of God was waiting in Noah’s days” (1 Pet. 3:19, 20), and to “the angels that sinned,” mentioned in connection with the “ancient world” of Noah’s time. (2 Pet. 2:4, 5)
For more, see:
What is an angel?
Angels - Why are they referred to as male in the Bible?
Angels - Who They Are
The Truth About Angels
Angels How They Affect Us
How Angels Can Help You
Do Humans Become Angels When They Die?
Spirit Creatures How They Affect Us
What Can Angels Do for You?
Does God Take Children to Become Angels in Heaven?
Do You Have a Guardian Angel?
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