What is the evangelical theory on the "Nephilim" or so-called "Sons of God" from Gn 6: 1-4?

From the Hebrew, one meaning of 'sons of Elohim' is beleivers in the true creator YHVH (who later became YAHOSHUA). Why the progeny of the match between a believer and a heathen would be mighty, is an unanswered question.