In this age of potpourri spirituality, Anique Olivier-Mason, 25, classifies herself generally as a Christian: she grew up Catholic and often attends a Presbyterian church near her home. But on a recent Friday night, she was attending Sabbath services at Larchmont Temple. Mrs. Olivier-Mason's husband, Joshua, is Jewish, and the couple became members of the synagogue, in Westchester County, last summer, committing to immersing themselves in the 800-family congregation. On this night, she stood by gamely as her husband, 25, bobbed, swayed and sang in enthusiastic Hebrew with others in the temple. With intermarriage so common, Reform synagogues like Larchmont...