Was it the act of seducing Eve to eat the apple that was his downfall, was he already fallen before that, or was it after?
closed as primarily opinion-based by curiousdannii, Matt Gutting, bruised reed, Dick Harfield, Mr. Beatitude Apr 20 at 15:19
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That happened before the Garden of Eden incident. Before then, he walked in heaven (on the holy mount of God).
He was actually cast down because he became proud.
Then he thought that he could be God, that he could take God's place in the universe as ruler of it:
That was his downfal: Pride--he thought he was as good as God.