D iane Knippers, a religious strategist who helped conservative Christians raise an increasingly loud and unified voice in the traditionally liberal bastions of mainline Protestantism in recent years, died on Monday in Arlington, Va. She was 53. The cause was cancer, according to the Institute on Religion and Democracy, a small but influential Washington group, of which Mrs. Knippers, a laywoman, was the longtime president. The institute is a resource center for conservative members of the mainline denominations - primarily the Presbyterian, Methodist and Episcopal Churches - disaffected by their churches' policies, especially on sexuality. When the Episcopal Church consecrated...