- Satan was in the garden in the form of a serpent (Genesis 3:1; Revelation 20:2).
- Since Satan is not God we know he was created and as all of creation, he was a part of the very good (Genesis 1:31).
- This one in the garden was a cherub. We know that in the garden were Adam, Eve, Satan, and God from Genesis 3. Neither Adam, Eve, or God are a cherub. Who does that leave?
The above comes from your web site in reference to whether or not Satan is a fallen angel. The above information appears to imply that Satan was a cherub. As far as I can tell according to Scripture this is incorrect. In Genesis 3:24, the cherubim was not placed in the garden until after Adam and Eve had been banished from the garden. So what Scripture is there that states a cherub was in the garden before and during the time of Adam and Eve's sin?
You are arguing from an absence of information. Genesis 3:24 does state that God placed cherubim in the Garden: "So He drove out the man; and He placed cherubim at the east of the garden of Eden, and a flaming sword which turned every way, to guard the way to the tree of life" (Genesis 3:24), but it does not say that this was the first time a cherub appeared in the Garden. All we know is that they were placed there to guard the way to the tree of life. In the allegory concerning the king of Tyre in Ezekiel 28:12-19 there is mention of a cherub being in the Garden who was perfect at creation but later sinned.
The point is not that there is a passage that says there were cherub in the Garden before or during Adam and Eve's fall. The point is that there is nothing ruling it out.
The original question was "Where in the Bible does the idea come that Satan is a fallen angel?" I merely assembled the evidence that has led people over the years to conclude that Satan was a fallen angel. The evidence is sketchy and indirect, but on the whole it appears to hint that Satan was once an angel who had fallen. While I cannot say for absolute certainty that Satan was a cherub, the evidence does appear to be pointing that way and I haven't found a hole. In addition, I know of no other explanation of the origin of Satan that matches the various hints in the Scriptures so well as this one.