Does the Bible provide any passages that might help in understanding how Satan became sinful? 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.

John
This seems like your main question:
&quot;But if God created Satan (originally) as a good (i.e., sinless) being who only entertained good thoughts, what was the original cause of Satan's evil or sinful thoughts?&quot;
That's tough to find a scriptural answer to because the first historically noted thing about Satan was that he was proud.

Isaiah 14:12,13 KJV How art thou fallen from heaven, O Lucifer, son of the morning! how art thou cut down to the ground, which didst weaken the nations! For thou hast said in thine heart, I will ascend into heaven, I will exalt my throne above the stars of God: I will sit also upon the mount of the congregation, in the sides of the north:
Luke 10:17-19 NIV The seventy-two returned with joy and said, “Lord, even the demons submit to us in your name.” He replied, “I saw Satan fall like lightning from heaven. I have given you authority to trample on snakes and scorpions and to overcome all the power of the enemy; nothing will harm you.

There's no historical details given about him before he succumbed to pride. This passage from James seems to say that it's one's evil desires that produce sin:

James 1:13-15 NIV When tempted, no one should say, “God is tempting me.” For God cannot be tempted by evil, nor does he tempt anyone; but each person is tempted when they are dragged away by their own evil desire and enticed. Then, after desire has conceived, it gives birth to sin; and sin, when it is full-grown, gives birth to death.

One could argue that evil desire causes pride, but you could say also that pride causes evil desire. God's word is intentionally ambiguous on this so one can't be dogmatic. And this still doesn't answer the question of how a sinless Lucifer could have gotten evil desires/pride in the first place.
But here's what we DO know:

God created Satan
Satan was tempted by power and...
Satan did succumb to that temptation.

God must have therefore created Satan with this ability to succumb to temptation, whether Satan was created sinless or not. Maybe God did all this to prove that anything outside his nature can succumb to evil, even his most gloriously created angel.