You’d think that in a film where a born-again Christian teenager gets pregnant, the comedy would write itself. But “Saved!,” the feature-film debut by Brian Dannelly and Michael Urban, can only mine so many jokes about fundamentalist Christians — and organized religion in general — before they become too earnest for their own good and wuss out on us. If you’re going to make a biting satire, keep biting until the film is over.

It is set at a Christian high school where the “coolest” kids in school are the most haughtily righteous ones. In this case, that’s Hilary Faye (Mandy Moore), a pretty, blond, sanctimonious girl who despises her brother, Roland (Macaulay Culkin). This might be because she’s generally a hypocrite, or it might be because she’s “righteously” frustrated with Roland for being a backslider: He hasn’t been thrilled with God, it seems, since he was in an accident that landed him in a wheelchair.

Their friend Mary (Jena Malone) is the one who becomes with-child, and not in nearly as sanctioned a way as her namesake. This Mary learned her boyfriend Dean (Chad Faust) was gay, and subsequently had a dubious vision in which Jesus told her to have sex with Dean in order to “cure” him. Dutifully, she did this, even though the vision came after being knocked unconscious in a swimming pool, and “Jesus” was actually the pool guy, diving in to save her.

Well, the sex thing didn’t work, and Dean’s fundamentalist parents send him to Mercy House, a rehab center for Christians struggling with a variety of tragic conditions, including homosexuality. Mary can’t figure out why Dean wasn’t cured when she did what she thought was right, and she’s even more alarmed to find herself expecting. Such behavior will not be looked upon favorably at a Christian high school, obviously.

The battle lines are soon drawn: Hilary Faye and her followers versus the more down-to-earth Christian kids, which include Mary, Roland, the pastor/principal’s son Patrick (Patrick Fugit), and Cassandra (Eva Amurri), the token Jew. Like all good high school movies, it has its climax at the prom, by which time Mary is “showing” rather noticeably.

The film has its share of funny lines and snarky situations — it’s like “Mean Girls” for the Christian set — and its cynical attitude toward religion is consistent. Much of its humor is laugh-out-loud outrageous. It’s the satire that’s slipshod: One minute we’re making fun of religious people, and the next minute we’re claiming to respect the idea of “faith.” Entertainment Weekly notes that critics have praised the movie “for walking a fine line between skewering ugly fundamentalism and respecting the faith behind it,” but it doesn’t come across that way to me. It seems to me to be mocking both religion and the people who believe in it, regardless of how fanatic they are about it, and the lip service paid to “faith” feels insincere.

More to the point, the last half-hour of the film isn’t funny, and barely tries to be. That’s where Dannelly and Urban (who wrote it together; Dannelly directed) try to wrap their jagged, snarling satire into a tidy finale, and it just doesn’t work. I can respect a movie that mocks things I believe in if it does it well, but not if it peters out before it’s finished.

C (1 hr., 32 min.; PG-13, a lot of profanity, one F-word, some sexuality.)