1 Enoch 6:1
1 And it came to pass when the population of humans had increased during those times, beautiful and attractive daughters were born to them. 2 And the angels, the children of heaven, saw them and desired them, and said to each other: 'Come, let us choose wives from among the humans and father children.' 3 And Semjâzâ, their leader, said to them: 'I fear that you will not actually agree to do this, and I alone will have to pay the penalty of a great sin.' 4 And they all replied to him and said: 'Let us all take an oath, and all bind ourselves with a solemn promise not to abandon this plan but to carry out this act.'
Lactantius Divine Institutes 2.15
When, therefore, the number of men had begun to increase, God in His forethought, lest the devil, to whom from the beginning He had given power over the earth, should by his subtlety either corrupt or destroy men, as he had done at first, sent angels for the protection and improvement of the human race; and inasmuch as He had given these a free will, He enjoined them above all things not to defile themselves with contamination from the earth, and thus lose the dignity of their heavenly nature. He plainly prohibited them from doing that which He knew that they would do, that they might entertain no hope of pardon. Therefore, while they abode among men, that most deceitful ruler of the earth, by his very association, gradually enticed them to vices, and polluted them by intercourse with women. Then, not being admitted into heaven on account of the sins into which they had plunged themselves, they fell to the earth. Thus from angels the devil makes them to become his satellites and attendants. But they who were born from these, because they were neither angels nor men, but bearing a kind of mixed nature, were not admitted into hell, as their fathers were not into heaven. Thus there came to be two kinds of demons; one of heaven, the other of the earth.
Notes and References
"... Perhaps the last major writer to embrace the Enoch-inspired exegesis of the passage was Lactantius (240-320 CE) in his Divine Institutes (written from 304-311). He begins his work by affirming divine providence and the unity of God but soon turns to pagan accounts of origins and the evil effects produced by the Greek and Roman cults. The second hook of the Divine Institutes focuses on worship of humans and celestial phenomena. As Lactantius tries to explain how such religions began, he bases himself heavily on the Watcher myth, though he fails to name the source from which he drew it. The relevant sections are book 2, chapters 14-17. It may seem that he places the angel narrative after the flood. He treats that event in chapter 13 and proceeds beyond it to trace ignorance of the deity to the descendants of the accursed Canaan and to charge the Egyptians with being especially inclined to worship the stars and to indulge in other forms of idolatry. However, he closes the thirteenth chapter by saying: 'Now let us return to the beginning of the world'. The opening words of chapter 14 leave no doubt that Genesis 6:1 underlies the report: 'When, therefore, the number of men had begun to increase ...' Lactantius indicates that God sent the angels to foil the devil, the ruler of the earth. They were to prevent him from corrupting humanity as he had at the first. In spite of God's warnings to the angels 'not to lose the dignity of their celestial substance through contagion with the stain of the earth,' the plan went awry ..."
VanderKam, James C. "1 Enoch, Enochic Motifs, and Enoch in Early Christian Literature" in VanderKam, James C., and William Adler (eds.) The Jewish Apocalyptic Heritage in Early Christianity (pp. 33-101) Fortress Press, 1993