Genesis 6:4

"The Nephilim were on the earth in those days - and also afterward - when the sons of God went to the daughters of men and had children by them.  They were the heroes of old, men of renown."  Genesis 6:4

There are several different possible translations of the word "Nephilim."  Many translations just transliterate as if this is the name of a people.  The Qbible translates it as "giants" but it also could mean "bully" or "tyrant."  In his book Old Testament Synonyms (page 54) Robert Baker Girdlestone points out that the word is derived from the form of "to fall" implying that these are men that used their power to cause other to fall.  From that "tyrants" actually seems like the closest translation to the truth.  Especially with the last sentence that these were "the heroes of old, men of renown."  So the children of the followers of God that married outside of their faith were the powerful men of their time that used their power to lead others further astray.