Although, I might already now have a provisional indirect answer to my original question ("What Caused Satan's Unbelief ...?") I have tried to reword that question, while simultaneously trying to maintain the original paragraph structure as much as possible.
I am a Christian, and I am aware of the grave theological dangers of "playing the Devil's Advocate" in the literal sense of the expression (and of attempting to defend sin and evil before God). However, I'm having difficulty avoiding perplexity about the cause of Satan's unbelief and pride. I know that it makes little sense to blame God (The Sovereign Creator) for the sins of His creatures (because sin is, by definition, disobedience of The Creator's [moral] Laws, and because there was no higher authority than God to overrule His decision to create anything). But Satan was originally created as a good (i.e., sinless) being who only entertained good thoughts. So, does the Bible offer any passages that might shed light on how we might discover the origin of Satan's unbelief in God's dictates?
By the way, I already read this question (posted at What could persuade a presumably otherwise-rational Satan to turn on God?) and the answers provided in that post don't address the issue that I am raising here in this post. I'm not asking why Satan (having already reached the point of indecision about whether it is rational to sin) would consider it prudent for him to proceed toward a state of actual sinfulness. My question goes further, and inquires about the origin of any deviation within Satan from his original state of sinlessness.