It occurs to me that I never responded to your explicit questions.

1. Should I have kept this to myself? What benefit does an irrational person have for confessing their irrationality? (Is this even possible? Is this post an attempted ploy?) I somewhat expect this post and the ensuing discussion to completely wreck my credibility as a commentator and participant.

I think it is fairly obvious that people's beliefspace can have great chasms beneath the sanity waterline while still containing valuable islands or continents of rationality. For my purposes, when asking for book recommendations and the like, I will discount yours to an extent on these grounds (or not, if they are in a specific domain where I consider religion irrelevant), but argument screens out authority, and you've proven your capacity to provide desireable (on the karma scale) commentary. Which leads to:

2. Presumably, there is a level of entry to LessWrong that is enforced. Does this level include filtering out certain beliefs and belief systems? Or is the system merit-based via karma and community voting? My karma is well above the level needed to post and my comments generally do better than worse. A merit-based system would prevent me from posting anything about religion or other irrational things, but is there a deeper problem? (More discussion below.) Should LessWrong /kick people who fail at rationality? Who makes the decision? Who draws the sanity water-line?

Were I setting the rules, the basic requirements for contributing to LessWrong would go along the lines of:

A. Don't be a jerk, as a rule (exceptions do occur).
B. Respect evidence, even if you defy it on occasion.
C. Respect valid reasoning, even if you cannot always articulate such for your positions.

If someone was (a) persistently mean/annoying/rude to others, (b) dismissive of the authority of observations, or (c) antipathetic to argument/debate/logic/etc., I would not want them wasting my time here. This no more excludes you (who has decided not to examine certain beliefs) than it does Mitchell_Porter (who has refused to accept as definitive the evidence for physicalism with respect to consciousness) or me (who has cast aspersions on the rationality of various LessWrong contributors).

These standards are far, far weaker than Eliezer Yudkowsky's sanity waterline, but I think they approximate the level where self-improvement in rationality becomes possible.

3. Being religious, I assume I am far below the desired sanity waterline that the community desires. How did I manage to scrape up over 500 karma? What have I demonstrated that would be good for other people to demonstrate? Have I acted appropriately as a religious person curious about rationality? Is there a problem with the system that lets someone like me get so far?

See my previous answer: you've demonstrated your ability to contribute in ways which the community approves of. (Examining your past comments supports this feeling.)

4. Where do I go from here? In the future, how should I act? Do I need to change my behavior as a result of this post? I am not calling out for any responses to my beliefs in particular, nor am I calling to other religious people at LessWrong to identify themselves. I am asking the community what they want me to do. Leave? Keep posting? Comment but don't post? Convert? Read everything posted and come back later?

Keep doing what you're doing - but keep "belief in God" in mind as a place where your beliefs differ from ours, and prepare (and actually do) have that discussion at some point or points in the future.