PHILADELPHIA — Lawyers for a Roman Catholic church official will demand his immediate release from prison Monday after an appeals court overturned his conviction in a novel priest-abuse case aimed squarely at the church hierarchy in Philadelphia.

Monsignor William Lynn, 62, is the first Catholic official ever prosecuted over his handling of priest sex-abuse complaints. He has served 18 months of the three- to six-year sentence handed down by a judge who said he helped predators remain in ministry, endangering new victims.

But the Superior Court threw out the conviction Thursday, ruling that the state’s child-endangerment law did not apply in the late 1990s to church supervisors like Lynn. The judge and the prosecutor “stretched the law too far,” said the Rev. Thomas Reese, a Jesuit theologian.