Step into St. Nicholas Cathedral on Massachusetts Avenue NW, and suddenly Washington's inside-the-Beltway world disappears. You are in the timeless milieu of Russian Orthodoxy, a special world that survived nearly a century of government oppression and came out stronger. A gray-bearded priest may be solemnly chanting, answered by a mixed choir with harmonies richer than you hear in most "official" Western church music. There is a hint of incense in the air, tapers burning all around you. Every inch of wall space is covered with holy images--life-size paintings of church fathers; saints; crucifixes; smaller icons in frames with attached sconces where candles glow. Worshipers stand, hushed and attentive, occasionally making a sign of the cross, touching the right shoulder before the left. Downstairs, a busy cafeteria serves simple but substantial meals of borscht and piroshki, pastry turnovers stuffed with meat or cabbage.

This was the perfect setting for "A Slavic Christmas," performed after yesterday's liturgy by Slavic Mosaic, a 15-member professional choir, expertly directed by Serge Boldireff, that specializes in classical and folk music of Slavic cultures. On this occasion (two days after the Russian Christmas and therefore solidly in the Christmas season), the group sang both liturgical works and folk carols from Russia, Poland, Ukraine, Belarus, Slovakia, the Czech Republic and Serbia.

Christmas comes later in Orthodox cultures than in the West, but popular devotion focuses on essentially the same elements, derived largely from the Gospels and particularly Saint Luke: the exultant song of the angels, the shepherds' simple veneration, the Virgin's maternal love. There were two formal, elaborate and richly emotive settings of the doxology "Slava Vyshnih Bogu" ("Glory to God"), recalling the angels' proclamation. But most of the program was devoted to carols--fresh, melodious and refreshingly unfamiliar to most non-Slavic Washingtonians, except for the popular Ukrainian "Carol of the Bells" and "Silent Night," which was sung as an encore, first in Russian and then (sing-along style) in English.

Slavic Mosaic sang with finely balanced and blended tone, rhythmic vitality and a sure sense of the music's varied styles. It is a relatively new organization but seems to be finding a special niche in the Washington scene. Next month it will sing in a gala concert performance of Tchaikovsky's opera "Iolanta" at the Russian Embassy, and later it is planning a tour of Russia. This may be a case of coals to Newcastle, but yesterday afternoon the chorus sounded ready to meet the challenge.