Relevance has been discussed before with respect to testimony on the blog. But, what if a prosecutor asks someone "Are you a Christian?" The next thing you should hear is the word "Objection!" coming out of the mouth of a Bucks County criminal defense lawyer.
The answer lies in Pennsylvania Rule of Evidence 610. This rules bars evidence of a witness' religious beliefs for the purpose of showing that his credibility is hurt or helped by his beliefs.
Also, under 42 Pa.C.S.A. § 5902, the statutory law prevents any witness from being questioned regarding their religious belief. The courts have further found that religious belief is irrelevant and prejudicial (see Commonwealth v. Myer from 1985 to start).
As you can guess, like anything in law, this rule is not absolute. In some cases, religion can be relevant. Commonwealth v. Covil allowed religious beliefs to be admitted to show a motive for killing.
As you can see, having your religious (or non-religious) beliefs mentioned in court is highly prejudicial and it does not further the trial.