In Genesis 6 we are instructed that "God saw the wickedness of man was great in the earth..."(6:5,6). And God grieved in His heart and repented that He had made man on the earth. "But Noah found grace in the eyes of the Lord"(6:8). What we have here is insight into a battle for a pure blood line to be preserved through which the promised seed of Genesis 3:15 would be born and thereby crush the serpent, destroying the power of sin, the curse, and the power of death. This "Seed of the woman" is the promised Messiah, who will be born of a pure virgin(Mary).
We see here in Genesis 6:1-4 "Satan's Plan To Corrupt All Flesh". If Satan can corrupt all the bloodlines through a mixture of fallen angels(the sons of God) and the daughters of man he can thwart the plan of God to destroy Satan and the power of sin, the curse, and death. Thankfully there was a man(Noah) and his family that was kept from this corruption. The reason for the worldwide flood and the destruction of all living flesh on the earth was to destroy all the unnatural offspring of the marriage of fallen angels with the daughters of men. Note that until this time mankind lived for hundreds of years, Methusaleh living the longest at 969 years. In verse 3 we are told that God would no longer tolerate man to live so long that they could generate such corruption again, so he decides to limit man's lifespan to a maximum of 120 years.
We are further instructed that this marriage of fallen angels with women also took place after the flood(verse 4). The hybrid offspring of the fallen angels are here called giants, and later are called nephilim and rephaim. These are the giants that were in the land of Canaan that Israel was commanded to destroy, every last one of them, because they were not humans, but hybrid offspring of the fallen angels.
Angels have the capacity to change dimensions, to leave their habitation as it were and transfer over to our dimension. Good angels can do this with God's permission as in the story of Lot in Genesis 18 where two angels are sent to Lot to destroy the sinful cities. We are told in Hebrews 13:2 that "we are not to forget to entertain strangers, because some have entertained angels unaware". Thus angels appear as humans when they cross over into our realm.
Note that the so-called "spiritual world" is not an unreal world, but another very real world, or maybe it is better to say, the same world but a different dimension. The invisible world(which God created just as much as the visible world - Col 1:16-18) is by definition simply "the unseen world". Invisible means "beyond eyesight". It does not mean "an unreal world".
The fallen angels that sinned with the daughters of men at the time of Noah were cast into tartarus(Hades) and are chained until the day of judgment(2 Peter 2:4,5). Jude reveals to us in His epistle, verses 6-8, that these fallen angels did not maintain their principality, but left their own habitation and committed fornication, going after strange(other) flesh. Church Father Justin Martyr(110-165 A.D.) is quoted, "But the angels transgressed...were captivated by love of women, and begat children".(Ante-Nicene Fathers Vol. 1, P. 190); and Church Father Methodius(260-312 A.D.) is quoted, "The devil was insolent...as also those angels who were enamored of fleshly charms, and had illicit intercourse with the daughters of men."(Ante-Nicene Fathers Vol. 6, p. 370). The fallen angels that did not sin in this fashion are still loose.
We know, from the Scriptures, more about the offspring(giants) of the fallen angels after the flood of Noah's day, than the the offspring(giants) that were on earth before the flood. Though it appears that there is evidence of civilizations all over the earth built by super strong and intelligent creatures that may pre-date the flood of Noah. Stonehenge, Easter Island, The Pyramids, etc.(More listed here soon).
We are instructed in Genesis 9:20 - 10:20 that it was through some perverse sin that Ham, the father of Canaan, committed against Noah that Satan gains entry into the bloodlines once again. For we see that Ham, who was cursed by Noah, has offspring that produces a "mighty one", Nimrod, who begins a kingdom that creates all the people groups that are later identified as giants and nephilim and rephaim. These are the descendants that Joshua and Caleb find occupying Canaan when Israel is sent to possess the promised land. The reason Isreal is told to kill "all" the people was because they were hybrids, giants, fallen ones(nephilim), and not just fellow humans.
In Numbers 13 we are instructed concerning the reality of "giants". The Lord tells Moses to send twelve men "to spy out the land of Canaan"(13:17). Upon their return they give their report, the land "surely flows with milk and honey...nevertheless the people be strong that dwell in the land...moreover we saw the children of the Anak there. The Amalekites dwell in the land of the south, and the Hittites, and the Jebusites, and the Amorites, dwell, in the mountains: and the Canaanites by the sea, and by the coast of Jordan"(27-33). Caleb speaks up and says, "we are well able to overcome it". But the men who went with Joshua and Caleb say, "We are not able...". They said, "it is a land that eats up its inhabitants, and all the people are men of great stature. And there we saw the giants, the sons of Anak, which come of the giants".
We read more of Israel's dealings with the giants: The Emims...a people great, and many, and tall, as the Anakims: which were also accounted giants, as the Anakims; but the Moabites call them Emims...the Horims(v. 12)...the children of Ammon...(That also was accounted a land of giants; giants dwell therein in old time; and the Ammonites call them Zamzummims, A people great, and many, and tall, as the Anakims; but the Lord destroyed them...he destroyed the Horims...and the Avims...were destroyed by the Caphtorims(Cretans)(19-23).
We continue to learn of the giants: "For only Og king of Bashan remained of the remnant of giants; behold his bedstead was...of iron...13 1/2 feet long and 6 feet wide. And this land...which was called the land of giants"(Deut. 3:11-13). In Deut. 7:1-5 we read of God's command to destroy all the inhabitants of the land He was sending them into to possess it; and to destroy their altars, break down their images, and cut down their groves, and burn their graven images with fire.(verse 5) All these things were used in Satanic worship and human sacrifices. Read more of the Anakims in Deut. 9:1-6.
In Joshua we read of "the remnant of the giants(Josh. 12:4; 13:12) and, of "the valley of the giants"(Josh. 15:8; 18:16), which interpreted is "the valley of the Rephaim"(2 Sam. 5:18,22; 1 Chron. 11:15; 14:9).
We learn of David slaying the giant Goliath, whose armour weighed 196 pounds and who was 9' 9" tall. (1 Sam. 17)
In 2 Samuel 21(1 Chron. 20:1-8/parallel) we learn of further wars that David and his mighty men had with the Philistines(giants) and in particular with the four sons of "The giant/Rapha"(verses 15-22). One giant son had a spear that the spearhead weighed 11 pounds. Another son of Rapha, had a staff like a weaver's beam. And yet another had 6 fingers on each hand, and 6 toes on each foot. These four sons were the brothers of Goliath of Gath.
In Judges 1:21-36; 2:1-5; 3:1-8 we read of Israel's disobedience in following God's command to kill all the inhabitants. Instead they chose to make money off the inhabitants with taxation(a tribute)(verse 28). But Judah obeyed the Lord, and the Lord was with Judah. And Caleb expelled the three sons of Anak(giants)(16-20). But Israel, once Joshua died(2:8-10), would serve Baalim and the groves(2:11; 3:7).
Why should we want to know about these events, and about the giants, and how they were introduced into the earth. For one, it is important to know our history as humans, and secondly, it is important to know our future, for Jesus said, "But as the days of Noah were, so shall also the coming of the Son of Man be. For as in the days that were before the flood they were eating, drinking, marrying and giving in marriage, until the day that Noah entered the ark. And knew not until the flood came, and took them all away; so shall also the coming of the Son of Man be."(Matthew 24:37-39) Is it possible that the same kind of evil perpetrated by fallen angels in the days of Noah's flood will be repeated in the days shortly before the Second Coming of Jesus Christ?