How do you determine what is true? Why do you believe what you do? Do you have an open mind to being wrong? Asking yourself these questions are crucial, before I present the following information. You might consider asking me the exact same questions myself; my responses would be through thorough investigation, presentation of verifiable facts, and an undoubted, yes.
Our timeless and omnipotent Creator, God Almighty, started everything we know in Genesis. The seven days of creation, Lucifer and his angels fall, the first sin of Adam and Eve – all monumental events in the inception of our world. What’s next to come is one of the many colossal events to have ever taken place.
Genesis 6: 1-9 (of 22)
“And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them, That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose. 3 And the Lord said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years. 4 There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown. 5 And God saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually. 6 And it repented the Lord that he had made man on the earth, and it grieved him at his heart. 7 And the Lord said, I will destroy man whom I have created from the face of the earth; both man, and beast, and the creeping thing, and the fowls of the air; for it repenteth me that I have made them. 8 But Noah found grace in the eyes of the Lord. 9 These are the generations of Noah: Noah was a just man and perfect in his generations, and Noah walked with God.”
Who are the “Sons of God”?
Perhaps while reading the previous scripture, you may have failed to notice the significance of it. After defining who the “Sons of God” are, we will discover the magnitude behind it. “Much debate has raged over the years over who were the sons of God and the daughters of men. Though some have suggested the sons of God were fallen angelic creatures who descended to earth and married women (daughters of men). However, there is no other scriptural basis for that. In fact, it would be biologically impossible. Moreover, Jesus indicated that the angelic realm is a sexless race, not having reproductive ability.” (Dr. David H. Sorenson, pastor & author)
The controversy defining who the “Sons of God” are, has been discussed for years without end. Two major theories presented, are the “Nephilim Theory” (Nephal in Hebrew is fall/cast down) and the “Sethite Theory”. The Nephilim theory is that the fallen angels (of Revelation 12:9) are the “Sons of God” in Genesis 6:2,4 and that they bred with women, creating giants (Nephilim). The Sethite Theory is that the Sons of God are referring to the righteous line of Seth (Adam’s youngest son) and the daughters of men are the wicked line of Cain (Adam’s eldest son).
Many have used Mark 12:25 as a crucial point towards the Nephilim theory being incorrect. “The verse only say that angels, being compared to specific humans, do not marry nor are given away in marriage. This refers to and compares the tradition of human men marrying and human women being given away but that does not, for one moment, indicate what form an angel can manifest. It only makes a point to say that they don’t marry, just like monks, certain priests, and others who take a vow not to.” 
(In Genesis 19 when the angels traveled with Lot in Sodom, the people of the city mistook them as men and in the book of John, when Mary Magdalene arrived at the empty tomb; she also mistook Jesus himself, accompanied by angels, as mere men.)
The use of Luke 17:27, questions, if the Nephilim theory were true, how the giants survived the flood; since there was never in the Bible a reference to a second incursion and the only survivors of the flood were Noah’s family. The answer to how the giants survived the flood will forever be a mystery.
You may think that since the above question has no irrefutable explanation, now, it must be untrue – and that is for you to decide. There are inconsistencies found in both. But as for the “righteous descendants of Seth” and “the rebellious daughters of Cain” it is not mentioned once in the entirety of the Bible regarding the Sethite theory. Defining who are the “Sons of God” genuinely determines which view must be interpreted. Entire churches have been split due to doctrinal or opposing views on how two parties understand the same exact scripture.
“Hebrew tradition in regard to history and recording events adheres to the principle of literary first mention. This means that the first time, chronologically, something is mentioned and defined semantically by context, that definition then becomes the meaning of that word until an obvious and stated alternative context is attached to that particular term. People don’t realize that the first time the term “sons of God” was mentioned was actually in the book of Job, not Genesis. Job is the oldest recorded book by authorship so when Moses compiled the Torah and edited Genesis, he would have possessed all previous holy scripture handed down over the generations. He would have most likely adhered to the prior terminology for authenticity and congruency. Job lived before Moses and he first used the term “sons of God” thereby defining it.” While the Sethite theory crumbles, the Nephilim theory shows potential.
Many have never heard both the views presented, and in the grand scheme of things, it doesn’t make much of an impact on us today. I believe that if the fallen angels did in fact create the abominations our Bible so frequently spoke of, then multiple questions we have in history can possibly be answered.
 Sorenson, David H. Understanding the Bible: An Independent Baptist Commentary. Duluth, MN: Northstar Ministries, a Ministry of Northstar Baptist Church, 2005. Print.
 Quoted by Tim Brock