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. Bultitude Apr 20 at 15:19
Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.
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.