In a world given to overstatement, proclaiming theatrical miracles is always risky business. But I'm pretty certain that the Roundhouse Theatre in Rockville has come up with one.

It's called "The Butterfingers Angel, Mary & Joseph, Herod the Nut & the Slaughter of 12 Hit Carols in a Pear Tree," a title that not only summarizes the evening's plot more or less, but also indicates some of the fanciful liberties that playwright William Gibson has taken with the traditional account of the Nativity. Written in 1975 to be performed in a Jesuit chapel in the Berkshires and just now getting its professional premiere in the Washington area, this "entertainment" (Gibson's description) is that rarity -- a deeply religious play that is moving without being mawkish, reverent without being pious. Most of all, it bursts with great antic fun.

Although God's work gets done, it does not get done easily. Mary -- "Maggie" to her 17 brothers, and every one of them a perfect Neanderthal -- is a headstrong woman, determined to avoid the shackles of motherhood. The angel, charged with the annunciation and attendant matters, is a bundle of ineptitude, who can't find the guidance he so sorely needs in his script. In fact, he has a terrible time with props in general, which seem to rebel at his touch. Joseph has a raging streak of jealousy and demands nothing less than divine reassurances of his wife's purity. The donkey is not up to his task ("Carry me," is his plaintive request, as Joseph piles on the burdens). Even the tree won't behave.

The tree, you see, is played by a gum-cracking chorine in a fur coat ("my bark," she points out, with the pride of a minor artiste), who really would prefer something more substantial to do than stand around waving her branches. Whenever, through sweet perseverance, the angel does succeed in getting the story moving along its usual tracks, a "man in grey" insinuates his way into the proceedings, sowing doubt and disenchantment among the cast. At one point, he turns up as Herod. ("Kneel, make yourself comfortable," he snaps at the Magi, then proceeds to play a demented concert on a battery of drums, fashioned, he reveals with rather ghoulish glee, from the skin of assorted relatives who once displeased him.)

God Himself might be expected to lend a helping hand, but God is noticeably silent in the crunch. As the angel explains it, fumbling once again with his script, there are two possibilities. If God does not exist, the universe is a gigantic story with no point or purpose. "We come and go in it like fruit flies." But if God does exist, even the fruit flies glow with the celestial grandeur of the stars.

The choice, Gibson implies, is ours to make. His faith rests on a literal interpretation of the old adage, "God helps those who help themselves." Each of us is, as the title of his most celebrated play has it, a potential "miracle worker." Gibson is not ready to take God's silence for His absence, but he realizes that to be on the safe side, someone has to make a reservation at the inn and get the water boiling.

With "The Butterfingers Angel," Gibson has pulled off the trickiest of marriages. His sensibility is that of the 20th century with all its inbred doubt. And yet his script is perfectly faithful to the spirit of those medieval mystery plays that humble workmen once put together for God's greater glory and their own robust amusement. There is the same mixture of faith and humor, of simplicity and mischief, of child-like joy and adult anxiety.

And at the Roundhouse, the youthful cast has hit upon the precise tone that brings it all to flower. The production has a decidedly improvisational look -- the ragtag costumes are purposefully anachronistic, and the little stages within the stage are constantly rearranged to suggest a pilgrimage in process. Indeed, the freshness of the events is as exhilarating as sea air. And yet director Jerry Whiddon has clearly regulated each wondrous moment with the loving precision of a master craftsman, for whom no detail is inconsequential.

The performances are both chaste and knowing at the same time. I can't remember ever seeing an actor who talked to God with such honesty as Michael Littman does. His Joseph is burly with strength and blunt with integrity. Greta Lambert makes Mary a no-nonsense creature, so straightforward and determined in her ways as to suggest that the liberated woman has in fact been around for ages. When she finally permits Joseph to kiss her, the stage seems lit by the pristine rays of paradise itself. Mark Jaster has the physical dexterity to make the angel a klutz of the first order. But more, he has the ineffable radiance of a tyke in God's candy shop. Ever disappointed that he can coax no more than a sorry bleat from his trumpet, he is ever eager to try anew, to make amends, to patch up the unpatchable.

David DiGiannantonio is threateningly understated as the "man in grey," which makes his excesses as Herod all the more telling. And as the tree, Gayle Behrman finds more attitudes and motivations in her role than Joyce Kilmer could have imagined. Actually, the whole cast constitutes a marvelous ensemble, their childlike ways avoiding the infantile while their naivete' deftly sidesteps the simple-minded. Best of all, their hearts respond fully to the story they are telling, even as it stumbles up against the hitches of the devil and the mishaps of the theater.

I doubt you will find another holiday production so wise, so entertaining, so gladdening as this one. It celebrates an age-old birthday party as if it had never been celebrated before. Cocking his ear at the strains of "Hark, the Herald Angels Sing" floating into his palace, Herod demands an explanation from one of his wives. "It's the latest song, your majesty," one of them replies. "Everybody is singing it." And in this grand little production, it does seem just that -- a spontaneous eruption of joy. Born of the moment. Never heard before. But certain -- for reasons as mysterious as fruit flies and angels who have trouble navigating a stage, let alone the head of a pin -- to soar right to the top of the charts. THE BUTTERFINGERS ANGEL. By William Gibson. Directed by Jerry Whiddon; musical arrangements, Chris Patton; sets, Douglas A. Cumming; lighting, Richard H. Young; costumes, Pamela MacFarlane; choreography, Cynthia Carter Pfanstiehl; with Mark Jaster, Gayle Behrman, Michael Littman, Greta Lambert, David DiGiannantonio. At the Roundhouse Theatre through Dec. 20.