The evidence is that this expression refers to spirit sons of God. But what evidence?

The first of those verses reads: “The sons of the true God began to notice the daughters of men, that they were good-looking; and they went taking wives for themselves, namely, all whom they chose.”​—Gen. 6:2.

In the Hebrew Scriptures, the expressions “the sons of the true God” and “the sons of God” occur at Genesis 6:2, 4; Job 1:6; 2:1; 38:7; and Psalm 89:6. What do those texts indicate about these “sons of God”?

“The sons of the true God” that we read about at Job 1:6 were clearly spirit creatures assembled in God’s presence. Among them was Satan, who had been “roving about in the earth.” (Job 1:7; 2:1, 2) Similarly, at Job 38:4-7, we read of “the sons of God” who ‘shouted in applause’ when God ‘laid the cornerstone’ of the earth. They must have been angelic sons, for humans had not yet been created. “The sons of God” mentioned at Psalm 89:6 are definitely heavenly creatures in God’s company, not humans.

Who then are “the sons of the true God” mentioned in the account at Genesis 6:2, 4? In harmony with the preceding Biblical facts, it is logical to conclude that the account is referring to spirit sons of God who came to earth.

Some find it hard to accept that angels might be interested in having sexual relations. Jesus’ words recorded at Matthew 22:30 show that marriage and sex relations do not exist in heaven. Yet, angels have on occasion materialized human bodies, even eating and drinking with men. (Gen. 18:1-8; 19:1-3) Hence, it is reasonable to conclude that in such a materialized form, they could have sexual relations with women.

There are Bible-based reasons for believing that some angels did just that. Jude 6, 7 compares the sin of the men of Sodom, who unnaturally went after flesh, with that of “the angels that did not keep their original position but forsook their own proper dwelling place.” A point in common was that both those angels and the Sodomites “committed fornication excessively and [went] out after flesh for unnatural use.” A similar passage in 1 Peter 3:19, 20 links disobedient angels with “Noah’s days.” (2 Pet. 2:4, 5) Consequently, the course that disobedient angels in Noah’s day took can be compared to the sin of Sodom and Gomorrah.

Such a conclusion makes sense when we recognize that “the sons of the true God” mentioned at Genesis 6:2, 4 were angels who materialized bodies and committed immorality with women.

The apostle Peter identifies these spirits as those who had “once been disobedient when the patience of God was waiting in Noah’s days.” (1 Pet. 3:20) Clearly, Peter was referring to spirit creatures who chose to join Satan’s rebellion. Jude mentions the angels who “did not keep their original position but forsook their own proper dwelling place,” saying that God “has reserved [them] with eternal bonds under dense darkness for the judgment of the great day.”​—Jude 6.

In what way were spirit creatures disobedient in Noah’s day? Before the Deluge, these wicked spirits materialized in human form​—something that God had not purposed for them. (Gen. 6:2, 4) Furthermore, those angels who had sex with women were practicing a perversion. God did not create spirit creatures to engage in sexual relations with women. (Gen. 5:2) These wicked, disobedient angels will be destroyed in God’s due time. For now, as Jude notes, they are in a condition of “dense darkness”​—a spiritual prison, so to speak.

When and how did Jesus preach to these “spirits in prison”? Peter writes that this occurred after Jesus was “made alive in the spirit.” (1 Pet. 3:18, 19) Note, too, that Peter says that Jesus “preached.” Peter’s use of the past tense suggests that the preaching occurred before Peter wrote his first letter. It seems, then, that sometime after his resurrection, Jesus made a proclamation to the wicked spirits regarding the fully justified punishment they are due to receive. It was not a preaching that held out any hope for them. It was a preaching of judgment. (Jonah 1:1, 2) Once Jesus had demonstrated his faith and loyalty to death and then was resurrected​—proving that the Devil indeed had no hold on him—​Jesus had the basis for making such a condemnatory proclamation.​—John 14:30; 16:8-11.

In the future, Jesus will bind and throw into the abyss both Satan and those angels. (Luke 8:30, 31; Rev. 20:1-3) Until that time, these disobedient spirits are in a condition of dense spiritual darkness, and their final destruction is certain.​—Rev. 20:7-10.