Nephilim and Rephaim
work in progress

Introduction

The word Nephilim and Rephaim are words describing the offspring of the Fallen Angels and the human race.  The word is often translated as giants in many Bible versions.  The Angels that participated in this are specifically identified as "Sons of God" but they are the Angels who rebelled and fell with Satan.  

They are described in Genesis as having sexual relations of some kind, which is not described, that created the offspring of giant men with often extraordinary strength.  The exact height is not given.  They are described as male and men of renown which is something we might equate in today's vernacular as heroes in the sense of something like a sports figure or a war hero.

Angel View of Genesis 6

Gen 6:2 That the Sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose. Sons of God in the Hebrew.   In Hebrew "Sons of God" is Bene HaElohim and it always refers to angels, Adam and Eve because Adam and Ever were direct creations of God, while we are not.  See Job 1:6, 2:1, 38:7; Lk 20:36
John 1:12

The term "Son of God" is always referenced to in the Bible as a direct creation of God.  For example there is of course the Angels, next there is Adam and Eve and then it is what all human beings aspire to become.   "But to all who did receive Him, who believed in His name, He gave the right to become Children of God."

Gen 6:4 The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of man and they bore children to them. These were the mighty men who were of old, the men of renown. 

The mingling of the Fallen Angels and the intermarrying with women was possibly with the entire human race.  The exception from the scriptures is Noah family before the Flood.  In Genesis 6:9 the Hebrew word "perfect" is "tamiym" which means without blemish, sound, healthful, without spot or unimpaired.  In light of the intermarrying by the Fallen Angels and Earth born women, Genesis 6:2 indicates this debauchery had not happened in his genealogy besides Noah being righteous.

Gen 6:9 These are the generations of Noah.  Noah was a righteous man, perfect in his generation. Noah walked with God. 

While the scripture does not give us the biological approach we cannot know the extent of Satanic knowledge and technology.     

In the beautiful majesty, remember that our Lord and Savior Jesus Christ entered the world through the virgin birth as prophesized in Isaiah 7:14 and the fulfillment in Mathew 1:20 "But as he considered these things, behold, an angel of the Lord appeared to him in a dream, saying, “Joseph, son of David, do not fear to take Mary as your wife, for that which is conceived in her is from the Holy Spirit."  God is Sovereign and certainly does everything perfectly.   The author mentions the Virgin Birth for the purpose of readers opening their minds to what the scripture states and not limiting our understanding.

There are three New Testament confirmations that are pointing to what the Fallen Angels were participating in Genesis.

Jude 1:6-7 6 And angels that kept not their own principality, but left their proper habitation, he hath kept in everlasting bonds under darkness unto the judgment of the great day. 7 Even as Sodom and Gomorrah, and the cities about them, having in like manner with these given themselves over to fornication and gone after strange flesh, are set forth as an example, suffering the punishment of eternal fire.   The word habitation in the Greek is oiketerion.  It the body we aspire to getting in Heaven.
2 Cor 5:1-2
(habitation)
1 For we know that if the earthly house of our tabernacle be dissolved, we have a building from God, a house not made with hands, eternal, in the heavens. 2 For verily in this we groan, longing to be clothed upon with our habitation which is from heaven.  So the fallen angels shed their habitation, heavenly body, in Jude 1:6 to participate in the debauchery. 
2 Peter 2:4-5 4 For if God spared not angels when they sinned, but cast them down to Tartarus, and committed them to chains of darkness, to be reserved unto judgment; 5 and spared not the ancient world, but preserved Noah with seven others, a preacher of righteousness, when he brought a flood upon the world of the ungodly;   We know that all of the fallen angels have not been cast into Tartarus but some have been cast in the ancient world.  It does not say specifically say why they were cast down but the link is the event occurred in the time of Noah, where Peter is telling us they suffered an immediate punishment.  This would be similar to when the Demons were afraid of Jesus casting them before their time and asked to enter the swine in Mark 5:12.
1 Peter 3:19-20 19 After being made alive, He went and made proclamation to the imprisoned angels 20 to those who were disobedient long ago when God waited patiently in the days of Noah while the ark was being built. In it only a few people, eight in all, were saved through water,   Note: that spirits is a misstranslation that meant angels in times past but currently has taken on the meaning of the spirit of man where in verse 19, the phrase "imprisoned spirits" should be "imprison angels".

Lines of Seth?

There is a "Lines Of Seth" theory, which has no scriptural collaboration, to explain away the Nephalim.  The theory is that the family line of Seth was not to marry the line of Cain and when they did then it created giants.  It started in the 5th century by Julius Africanus to explain away the Fallen Angel view of creating Nephilim and was later embraced by Ausutine.   Do your own research, if needed, on this theory.  It is the opinion of this author that it easily proven wrong and it is not worth spending too much time on.  In brief, here are the points to prove it is wrong:

.... continue work here ...

It also after the Flood specifically to a geographic area of the Promised Land and Jesus eludes it will happen again before His Second Coming. 

     
     

The Nephim are not eligible to receive the Gift of the Cross.

Isaiah 26:14 They are dead, they shall not live; they are Rephaim(/Nephilim), they shall not rise: therefore hast thou visited and destroyed them, and made all their memory to perish.  Note that Rephaim appears to be a synonym Nephilim.  Additionally some scholars have suggested that the Rephaim may be descendants of the Nephilim where the genealogy may be cleansed of the curse after the 10th generation.

The spirit of the Nephim do not have a body and might refere to Demons which is distinctly different than the Fallen Angels.