In the 15th Century it was estimated that 133,306,668 Angels fell from the heavens in a total of 9 days according to Cardinal Bishop of Tusculum (1273), and reaffirmed by Alphonso de Spina (c. 1460). I bet this number astounds even the most open of minds. I for one did not realize the numbers until I started to research the topic.

By some accounts, it is said that Lucifer [Satan] was very much angered by God's love for Adam. This bred contempt and jealousy, hence delving into the sin of Pride (Ego). Lucifer [Satan] then in turn declared himself to be a great God and led the rebellion, with the help of some angels that he deceived into thinking he [Lucifer/Satan] was right and God was wrong. Defeated, Lucifer [Satan] and his band of angels were "thrown" out of heaven and ever since have waged war against God by trying to destroy God's Creation--the Earth.

The reason why Lucifer and Satan are both listed as the angel that started the battle with God is due to a misreading of the passage Isaiah 14:12 [See Lucifer and Satan section]. Hence because of this misreading these two angels have been interchanged with each other over centuries.

The Old Testament does not mention the notion of fallen angels whatsoever. The actual first reference to a fallen angel or the fallen angels does not show up until the New Testament, specifically Revelation 12--

"And his [the dragon's or Satan's] tail drew the third part of the stars of heaven [angels] and did cast them to earth...and Satan, which deceiveth the whole world; he was cast out into the earth and his angels were cast out with him."

In the writings of Enoch 1, there seems to be about 200 that fell, naming about 19 (spelling variations observed) and listing the most prominent among them--Semyaza, Azazel, Sariel, Rumiel, Danjal, Turel, Kokabel.

In the Book of Enoch it is mentioned that a group of angels lusted after mortal women. Another "No-No" sin. There is a very brief excerpt about this group in Genesis 6:2-4--

"That the sons of God saw the daughters of men, that they were beautiful; and they took wives for themselves of all whom they chose."

There is a group called the Watchers or Grigori. This is the group that this passage speaks of. They are said to look like the human male, but taller/larger, they never sleep, and are silent. In Jewish legend/lore it is said that there are good and evil watchers, the good watchers residing in the 5th Heaven, while the evil ones reside 3rd Heaven.

The Watchers fell when they started to take mortal women as wives, and created "offspring" from these unions. The Church in earlier times dismissed this notion stating that "angels cannot have relations with human beings."

These are only few ideas of how the angels did indeed fall from the heavens. There are so many theories from many different areas of study, too many to even attempt to post them all here on this page.