There is no verse in the Bible that says, “A third of the angels fell from heaven.” The idea that, when Satan rebelled against God, one third of the angels went with him comes from certain other verses that, when put together, suggest that’s what happened.
On the sixth day of creation, God declared everything to be “very good” (Genesis 1:31). We assume the “everything” includes angels. Sometime after that, Satan rebelled against God and was cast out of heaven. Jesus witnessed the event: “I saw Satan fall like lightning from heaven” (Luke 10:18)—an event that the Lord uses to explain the exorcisms performed by the 72 (verse 17).
Another passage that also seems to describe the fall of Satan from heaven is Isaiah 14:12–15. The context is primarily about a human king, as judgment is pronounced against Babylon. The poetic language used by Isaiah is so grand, however, that many scholars have concluded that there is more to the passage. References to “the whole earth” (Isaiah 14:7), the king’s “fall from heaven” (verse 12), his desire to exalt himself “above the stars” (verse 13), and the symbolic name Lucifer or Light-bringer (verse 12) are all expressions of hyperbolic greatness. If the intended subject is only the human king of Babylon, then everything is figurative (and greatly exaggerated); however, if there is a secondary spiritual character in view, then the descriptions could be quite literal. God is pronouncing judgment on both “kings of Babylon”—the human king and the spiritual potentate who empowers him (Babylon being a symbol of rebellion from Genesis to Revelation). The wonderment of Isaiah 14:12 (“How you have fallen from heaven, O morning star, son of the dawn! You have been cast down to the earth, you who once laid low the nations!”) could indeed have a dual interpretation.
Further, the book of the Revelation describes “a star that had fallen from the sky to the earth” (Revelation 9:1). This “star” is given a key to the bottomless pit and proceeds to release a horde of tormenting “locusts” (Revelation 9:2–11). While Revelation 9:1 is not a clear reference to Satan, it could be: the fact that the “star” that fell from heaven has a key (authority) is no problem, especially since he uses the key to unleash terror upon the earth.
So, the Bible explicitly and implicitly teaches that Satan fell from his position in heaven. But how do we know that one third of the angels also fell with him?
The passage that best supports the idea that Satan took one third of the angels with him in rebellion is Revelation 12:3–4. John sees a sign in heaven: “An enormous red dragon with seven heads and ten horns and seven crowns on its heads. Its tail swept a third of the stars out of the sky and flung them to the earth.” John then relates that the dragon was hurled down to the earth and positively identifies it as “that ancient serpent called the devil, or Satan, who leads the whole world astray” (Revelation 12:9). The dragon is not thrown to earth alone, however: “His angels were cast out with him” (verse 9). The angels that are ejected from heaven with Satan we associate with the “third of the stars” that the dragon’s tail swept from heaven to earth in verse 4.
If the “stars” of Revelation 12:4 are indeed a symbolic reference to Satan’s “angels” in verse 9, then what we have is a reference to the fall of a portion of the angels (the heavenly hosts) who followed Satan in his rebellion. Two thirds of the angels remained loyal to God and are called the “holy angels” in Scripture (e.g., Mark 8:38); one third of the original angels joined Satan and are called “unclean spirits” or “demons” today (e.g., Mark 9:25).