From this question: [Does the Bible provide any passages that might help in understanding how Satan became sinful?](https://christianity.stackexchange.com/questions/30499/does-the-bible-provide-any-passages-that-might-help-in-understanding-how-satan-b/30985#30985), answers have attempted to find from a biblical-basis, how Satan became sinful.

This question is asking what God himself says was the sin of the fallen angels. The earlier question deals with how, this one: *According to God, **what** was the sin of the fallen angels?*