The Sons of God were described in the Old Testament as fallen angels who did not keep their proper domain, came to earth took wives on earth and the children born from this union were wicked giants in the land.

• And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them, Genesis 6:1
• That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose. Gen. 6:2
• 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. Gen. 6:4 

​These angels who left were compared to Sodom and Gomorrha:

• And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day. Jude 1:6
• Even as Sodom and Gomorrha, and the cities about them in like manner, giving themselves over to fornication, and going after strange flesh, are set forth for an example, suffering the vengeance of eternal fire. Jude 1:7
• Likewise also these filthy dreamers defile the flesh, despise dominion, and speak evil of dignities. Jude 1:8
• Jude 1:9 Yet Michael the archangel, when contending with the devil he disputed about the body of Moses, durst not bring against him a railing accusation, but said, The Lord rebuke thee. Jude 1:9
• But these speak evil of those things which they know not: but what they know naturally, as brute beasts, in those things they corrupt themselves. Jude 1:10

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God cast the angels that sinned down to hell in chains of darkness to be reserved for judgement:

• For if God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment; 2 Peter 2:4
• And spared not the old world, but saved Noah the eighth person, a preacher of righteousness, bringing in the flood upon the world of the ungodly; 2 Peter 2:5
• And turning the cities of Sodom and Gomorrha into ashes condemned them with an overthrow, making them an ensample unto those that after should live ungodly; 2 Peter 2:6
• And delivered just Lot, vexed with the filthy conversation of the wicked: 2 Peter 2:7
• (For that righteous man dwelling among them, in seeing and hearing, vexed his righteous soul from day to day with their unlawful deeds;) 2 Peter 2:8
• The Lord knoweth how to deliver the godly out of temptations, and to reserve the unjust unto the day of judgment to be punished: 2 Peter 2:9
• But chiefly them that walk after the flesh in the lust of uncleanness, and despise government. Presumptuous are they, selfwilled, they are not afraid to speak evil of dignities. 2 Peter 2:10
• Whereas angels, which are greater in power and might, bring not railing accusation against them before the Lord. 2 Peter 2:11

What Jesus said about the angels of God:

• ​Matthew 22:29 Jesus answered and said unto them, Ye do err, not knowing the scriptures, nor the power of God. 
• Matthew 22:30 For in the resurrection they neither marry, nor are given in marriage, but are as the angels of God in heaven.
• Matthew 22:31 But as touching the resurrection of the dead, have ye not read that which was spoken unto you by God, saying,
• Matthew 22:32 I am the God of Abraham, and the God of Isaac, and the God of Jacob? God is not the God of the dead, but of the living.

​Christ gave all people, the ability to become Sons of God
After Christ came to Earth and was sacrificed to save us from our sins all people were given the opportunity to become the Sons of God.

• Whosoever believeth that Jesus is the Christ is born of God: and every one that loveth him that begat loveth him also that is begotten of him. I John 5:1
• To redeem them that were under the law, that we might receive the adoption of sons. Galatians 4:5