Interesting points.
I once heard a minister (Ben Jordan from Missouri) speak about a wealthy man who was going to hire a driver. He found the three best candidates and asked them to test drive him around a set of directions as he sat in the back.
The first driver was doing well until he came upon a curve on the side of the mountain. He wanted to impress the man, so he accelerated around the curve as closely as he could to the edge. When the ride was over, the rich man remarked, "Wow -- you were very close to the edge during that curve and I hardly felt it."
The second driver was driving well until he saw the same curve and the fresh tracks near the edge. He realized that the previous candidate had approached the curve very closely; so, he decided to drive even closer. He accelerated around the curve next to the edge of the road and cliff -- even closer than the previous candidate. When the ride was completed, the rich man remarked how close this driver got to the edge.
The third and final driver was also driving extremely well when he saw the same dangerous curve and the two sets of tracks that went very near the cliffs edge. He thought to himself, "I want to really impress this driver."
So what did he do?
He stayed as far away from that curve as he possibly could while staying in his own lane! And, of course, he was the driver that the rich man hired. Why? Because, the third driver was the one who stayed furthest away from danger!
I think that this is an important principle to remember when it comes with people living their lives with "freedom" under Christ. They often use their freedom to get close to danger...thinking that their "security in Christ" will keep them out of (spiritual, mental, physical, emotional, relational) harm's way. Like DEADn said, they literally set themselves up for "failure" by doing things, going places or getting close enough to the "edge."
Now, the "edge" doesn't have to be a blatant sin. It can be a dangerous "omission" (failure to spend time with Christ or His Word). During college (at a secular school), I knew a few Christians who were majoring in Human Biology. Two of them wanted to eventually go to medical school and become medical doctors.
During a Bible study on campus, I was telling them the importance of staying in communication with the Lord through prayer and study of his Word. I explained how we are often taught things that contradict God's Word, so we must counter it by remaining steadfast and diligent through fellowship with the Lord. One of them was a new believer and eagerly took that advice. The other was a long-time Christian, raised in church, and very active with Christian activities (including missionary and evangelical activities). She wanted to become a medical doctor because she wanted to utilize such skill on the mission field.
Sadly, the long-time Christian eventually took several wrong turns in her life. She started privately believing in evolution...and then almost tried to justify the theory by saying that it doesn't conflict with Biblical creation. Eventually, she stopped going to Bible studies, then church and started hanging out with the "wrong" people from fraternities and sororities. People tried to warn her, but she just kept moving closer and closer to the edge. During her junior year, she became impregnated from a casual relationship with an athlete who didn't even want to maintain a serious relationship with her (baby or not).
She spoke with me about this (before her parents even knew) because she wanted prayer and advice. The impact of her pregnancy and the fear associated with it (and the fear of others finding out) caused her to run back to Christ. Thank God that she realized that abortion was NOT an option. Eventually, she told her parents. She dropped out of school and we never saw her again.
However, the other girl (the new believer) -- who was just as intelligent and gifted -- remained steadfast in seeking the Lord. Both girls attended the same classes, but only the one who stayed in the Word and stayed away from bad worldly associations (whenever possible) kept her testimony. She kept in contact with my wife. We found out that her entire family came to Christ. This young woman went on to medical school and married a man from her home church.
This may have been a bit of an extreme anecdote. Of course, like DEADn said, there are also individuals who are doing and saying all of the "right" things yet still spiral out of what is right for other reasons. I suppose that the reasons may be just as mentioned before, but I believe that the underlying principle is still the same: Remain steadfast in prayer and God's Word. That communication with Christ is vital in maintaining our faith -- because it is exactly what our "faith" is in that we trust in Who we cannot see.
I think that we all go through experiences on both mountaintops and valleys that are as egregiously disheartening as the "valley of the shadow of death" mentioned by David. When I experience those "seasons," I do my best to share my testimony with someone (usually an unbeliever). I have felt my passion and love for Christ rekindled when I share my testimony with others.
I suppose that this is because there is a joy and power associated with ministering to someone. It makes me think of the encouragement that Paul received as he shared his testimony while standing trial (at least twice). There is an encouragement that cannot be felt in any other way that sharing Christ -- and how we came to know Him -- with others. However, I also think that it also serves as a personal reminder (by listening to our own words) of what Christ has done for us...and what He has taken us through.
So, whenever I feel that "disappointment," I make the extra effort to find someone and share Christ with them. Sometimes, it might be online; but, most of the time, it is in person. Words cannot express how "strengthened" that I feel when that happens.
Anyway, thanks for sharing these thoughts. I agree that this disillusionment is a common thing in many, many believers and that the best way out is by steadfastly maintaining open (and HONEST) prayer with the Lord, reading His Word and sharing our testimony with others.