Father Sergius Book

''Father Sergius,'' the 1978 Russian film that opens today at the Film Forum, is like its title character -sorrowfully handsome, passionate and unworldly. It's Leo Tolstoy's posthumously published novella, adapted and directed by Igor Talankin, about a fearfully self-righteous aristocrat, Prince Stepan Kasatsky (Sergei Bondarchuk), and his lifelong search for spiritual perfection.

As a young officer in the service of Czar Nicholas I, the prince is as demanding of others as he is of himself. He places honor above all and, as he says, worships the Czar as a god. On the eve of his marriage to a beautiful, equally young countess, whom he idealizes as an earthly representation of purity, she confesses that she was once the mistress of the Czar.

Other ambitious officers might accept this as a kind of blessing, an unexpected advantage at court. Kasatsky, instead, is horrified. He abandons his fiancee, his estate and the worldly life to enter the church.

Most of the film is concerned with the efforts of the prince, now Father Sergius, to understand the roots of his behavior, which, he eventually sees, was prompted by anger instead of faith. Still, he has the need to be the best at whatever he does. Over the years, Father Sergius becomes famous as a holy man, a hermit who lives the ascetic life and, at the door of his cave, dispenses wisdom to pilgrims - to the church's satisfaction and financial profit. He finally realizes that this life, too, is as false as the one he gave up as a young man.

The Tolstoy text is translated to the screen with almost paragraph-by-paragraph fidelity, which gives the film a curious, uncinematic stateliness. The pacing is deliberate, more at the speed one reads than the way one expects to follow things on the screen.

Mr. Bondarchuk is majestic rather than moving, but this is probably inevitable because the furies that are attacking Father Sergius are, with two exceptions, unseen. Only toward the end does Mr. Bondarchuk, one of the most popular Russian directors (''War and Peace'') as well as actors, have an opportunity to dramatize the character in a way the screen understands.

Among the members of the supporting cast, the standout is Alla Demidova, who plays the small but important role of Father Sergius's aristocratic, once beautiful cousin, whose life becomes a sort of object lesson for the former prince.

''Father Sergius'' is a film that's entirely at the service of written literature.

*The Cast

FATHER SERGIUS, directed by Igor Talankin; screenplay (Russian, with English subtitles) by Igor Talankin, based on a novel by Tolstoy; photography, Georgi Rerberg and Anatoly Nikolayev; music by Alfred Shnitke; produced by the Mosfilm Studio. At the Film Forum 2, 57 Watts Street. Running time: 99 minutes. This film is not rated.

Prince Stepan Kasatsky-Father Sergius . . . . . Sergei Bondarchuk
Countess Mary . . . . . Valentina Titova
Emperor Nicholas 1 . . . . . Vladislav Strzhelchik
Seductress . . . . . Ludmila Maksakova
Pashenka . . . . . Alla Demidova
Lady at Court Ball . . . . . Irina Skobtseva