An evening of Christopher Durang plays

Another play coping with a crisis of faith is Christopher Durang's ''Sister Mary Ignatius Explains It All for You,'' at the Charles Playhouse through Oct. 26.

Sister Mary Ignatius (superbly played by Elizabeth Franz) is a parochial-school teacher whose explanations of Roman Catholicism are hilariously illogical (Mortal sin: ''Murder, sex outside of marriage, and hijacking planes''), and who both terrorizes and sweetly charms her students. Her current pet, seven-year-old Thomas, parrots catechism on cue, for which he is rewarded, like a seal, with cookies.

The crisis occurs when four of Sister Mary Ignatius's former students (class of '59) show up. Their aim: retaliation for her rigid teachings, which have left them floundering as adults and coping with abortions, homosexuality, and alcoholism. In the surprise ending, Sister Mary shows her rule is still absolute.

Many Catholics are offended - to the point of picketing this play, which starts out by cracking big jokes at their religion and ends in an indictment of it. Even Boston Mayor Raymond Flynn was displeased.

The questions the play asks, however, could just as easily be asked of other religions and institutions. Do its teachings provide a framework or a prison for faith? Do they give satisfying answers to questions of the heart or hide behind the shield of dogma? Is questioning seen as a sign of growth or heresy?

This funny, piercing play asks some good questions, but Durang falls short of his potential. Staying at the level of flat-out criticism, he offers no solutions, answers, or alternatives. And in so doing, he leaves us in as much of a void as he accuses the church of doing.

On the same bill is ''The Actor's Nightmare,'' a real stitch of a playlet about a man innocently thrown onstage in the wrong costume, not knowing his lines, his fellow actors, or even the play.

The situation is a born laugh track, waiting to be laughed. Jeff Brooks is wonderful as the initially terrified and later cocky young actor-on-the-spot, and so are the other actors in snippets from ''Private Lives'' to ''Hamlet.'' The timing is lickety-split, and the sense of ensemble fluid. It's a delightfully sily show.