Though many argue over their identity, Scripture is not as vague as that would lead you to believe. This word is first found in Genesis 6:4 and twice in Numbers 13:33. The word “Nephilim” means “fallen ones” or “overthrown ones.” It is used in reference to fallen angels (some argue they are the righteous line of Seth that fell.)
A quick look at the biblical accounts makes it clear that the intention is for the reader to understand them to be fallen angels. Genesis 6 teaches us that the Nephilim were a product of sexual relations between the “sons of God” and the “daughters of men.” Numbers 13 teaches us that these Nephilim were giants. If we are to argue that these are the outcome of a union between Seth’s line and Cain’s line, three problems arise.
First, “sons of God” is a phrase used only of angels until after Christ’s death wherein believers are adopted and become sons of God. Adam, in Luke, is referred to as the son of God. This indicates that “sons of God” refers to those beings who are a direct creation of God, as opposed to those who are created through a physical birth. Second, it is a matter of historical record that the ancients understood this to refer to angels. Third, why would the product of a righteous man and an unrighteous woman result in a giant child? Why, then, wouldn’t the product of a righteous man and a righteous woman result in an even greater child?
Most object to the angel view not because of Scripture but because of their own natural aversion to what the text naturally says. Two facts support this, though. First, Scripture is clear that angels can take on human appearance and flesh (Gen. 18-19:1, Heb. 13:2). Second, mankind itself breaks God’s laws on the limits or boundaries of a species, so it is not unthinkable that angels (whom we know did rebel against God) could do so as well.