United States Catholic leaders yesterday welcomed Pope John Paul II's surprise announcement of an extraordinary synod of Catholic bishops from around the world to be held later this year to evaluate the reforms of the Second Vatican Council of two decades ago.

"I think it's a good idea for any institution, religious or secular, to step back and reflect on what has happened . . . whether the new ideas you've been trying to implement really bring us closer to your goal," said Cardinal Joseph L. Bernardin of Chicago.

The pontiff's announcement of the synod, scheduled in Rome from Nov. 25 to Dec. 6, came as a complete surprise to officials at the headquarters of the United States hierarchy here, who said they had had no inkling of the plans. Even Bernardin, one of the most influential prelates in this country, said in a telephone interview that he had no advance knowledge of the plans.

John Paul II said he was calling the synod "above all to relive in some way the extraordinary atmosphere" of Vatican II and "to exchange and discuss in greater depth, experiences and news as to the application of the council at the level of the universal church and of particular churches; to encourage further study and the constant inclusion of Vatican II in the life of the church and in the light of new demands."

The announcement, late Friday night on the eve of the pope's 12-day Latin America tour, was made at the conclusion of a mass ending the Week of Prayer for Christian Unity, a concern that received a great deal of momentum from Vatican II.

With his characteristic sense of the dramatic, the pontiff announced the special synod from the same altar at the Basilica of St. Paul's Outside the Walls where, exactly 26 years before, Pope John XXIII had announced -- equally unexpectedly -- the summoning of Vatican II.

The Second Vatican Council produced massive changes: a new liturgy that substituted the language of the people for Latin in the mass, the beginning of unity talks with other Christians and a cooperative stance toward other religions, and the encouragement of lay Catholics to take increased responsibility for the church and for social action.

In his brief remarks Friday night about the proposed synod, the pope noted that Vatican II has been a "constant point of reference" of his pontificate and "remains the fundamental event of the contemporary life of the church." Msgr. John Tracy Ellis, dean of American Catholic historians, pointed out that as a young Polish bishop, John Paul was "one of the main architects of the council" and was "quite enthusiastic in promoting the work of the council."

But observers have seen some of the pope's actions as moving away from the council's reforms. Last year John Paul authorized use of the old Latin mass under certain conditions. A Vatican-ordered study of American nuns was viewed by many as counter to the spirit of Vatican II.

And while the pope continues to call for greater concern for the poor, he approved a Vatican denunciation of liberation theology -- the idea that Christians are obligated to fight political or economic structures that hold people in oppressive circumstances -- so harsh that his own secretary of state publicly disagreed.

Aboard the Caracas-bound papal aircraft, Cardinal Agostino Casaroli, the Vatican secretary of state, said Rome already had made its analysis of possible changes in Vatican II, United Press International reported.

"We in the Vatican have made our diagnosis," he said. "Now we have to see what the different bishops of the world have to say." The cardinal said the extraordinary synod was "the personal idea of the pope."

Bernardin warned against efforts to "politicize" the session. "There are already people reading into it their own hopes and anxieties," he said.

By church law, synods of bishops are technically advisory to the pope and cannot override or overturn decisions of a Vatican council.