The U. S. Court of Appeals here yesterday quickly rejected a plea by atheist Madalyn Murray O'Hair that Pope John Paul II be blocked from conducting a mass Sunday on the Mall.

The argument on O'Hair's suit, which claims church-state lines are entangled when the government allows a religious event to be performed on public property, was over at 11:15 a.m. At 4:45 p.m., U. S. Circuit judges Harold Leventhal, George A. MacKinon and Patricia M. Wald issued their written opinions, unanimously rejecting O'Hair's argument.

Even before the court of appeals acted, O'Hair's attorneys had already gone to the Supreme Court asking for a ruling. No action had been taken on that request by late yesterday.

Laventhal wrote for the appellate panel that "it is an important function of government to permit large assemblies in outdoor parks by our citizens."

He noted that other religious and nonreligious groups had previously been given permission for activities on the Mall, and that the mere approval of a Mall demonstration did not indicate any government approval of its content or sponsors.

He said the only message being given by the government in allowing the celebration of the mass "is approval of the principle of freedom of demonstration, for all groups, for all religions, even for those opposing religion."