The largest religious assembly space in the country is the recently completed Conference Center of the Church of Jesus Christ of Latter-Day Saints in downtown Salt Lake City. It was built to accommodate 21,000 people for the Semiannual General Conference of church members, but it also houses the Mormon Tabernacle Choir and is used for church pageants. The approach of the architects, Zimmer Gunsul Frasca of Portland, Ore., shows the influence megachurches have had on mainstream religions. The ecclesiastical imagery is confined to the giant pipe organ. The arena seating, the mainstream decor, the profusion of lighting and television broadcasting equipment, as well as the surrounding lobbies and vestibules, are distinctly secular. If you can't beat 'em, join 'em.