1 Enoch 6:2

1 And it came to pass when the children of men had multiplied that in those days were born unto them beautiful and comely daughters. 2 And the angels, the children of the heaven, saw and lusted after them, and said to one another: 'Come, let us choose us wives from among the children of men and beget us children.' 3 And Semjâzâ, who was their leader, said unto them: 'I fear ye will not indeed agree to do this deed, and I alone shall have to pay the penalty of a great sin.' 4 And they all answered him and said: 'Let us all swear an oath, and all bind ourselves by mutual imprecations not to abandon this plan but to do this thing.' 5 Then sware they all together and bound themselves by mutual imprecations upon it.

Date: 200-50 B.C.E.
* Dates are based on scholarly estimates

Eusebius Preparation for the Gospel 5.4

And this argument is still further confirmed by Plutarch, in the passage where he says that the mythical narratives told as concerning gods are certain tales about daemons, and the deeds of Giants and Titans celebrated in song among the Greeks are also stories about daemons, intended to suggest a new phase of thought. Of this kind then perhaps were the statements in the Sacred Scripture concerning the giants before the Mood, and those concerning their progenitors, of whom it is said, 'And when the angels of God saw the daughters of men that they were fair, they took unto them wives of all that they chose,' and of these were born 'the giants the men of renown which were of old.' For one might say that these daemons are those giants, and that their spirits have been deified by the subsequent generations of men, and that their battles, and their quarrels among themselves, and their wars are the subjects of these legends that are told as of gods.

Date: 305-315 C.E.
* Dates are based on scholarly estimates