A month-long battle for the airwaves seemingly pitting church against state is resolving itself in favor of the former. Sister Janet Mead's rock'n'roll rendering of "The Lord's Prayer" is nosing past Byron MacGregor's flag-waving narrative paean "The Americans" and ascending to the top of the radio play-charts.

Only occasional bursts of theological protest are still to be heard amidst general acclaim for the 36-year-old Australian nun's tuneful testament of faith. "Stop playing that goddamn 'Lord's Prayer,' " brayed one elderly woman, clearly outraged by the encroachments of rock upon her religion, in a phone call to a Detroit disc jockey. "All I did was sing a little song," rejoins Sister Janet, a member of the Order of Mercy under whose aegis she teaches music and drama at Adelaide's St. Aloysius College.

Her recording was conceived simply as a way of sharing with ecclesiastical colleagues some of the liturgy of the youth-oriented rock masses which had gained her a weekly following over Australian radio. No one was more surprised than Janet when her "little song" became an international success. Though she had a conservatory education in piano and violin before taking the veil at 17, the nun has not escaped music critics, including those of the cloth. "It's bad rock'n'roll," contends New York's hip Father Peter Madori of the boom-boom wah-wah accompaniment that does somehow seem more dated than the ageless lyric it conveys.

Sister Janet, fresh from a visit to Europe in which she studied musical trends in religious institutions but didn't perform, remains undisturbed. The Australian Broadcast Company has invited her to spread her ministry to television. Of the money redounding to the Order of Mercy from her record sales, she remarks in the same squeaky voice in which she intoned "The Lord's Prayer," "It will be handy to have."