The U.S. Roman Catholic bishops reverted to their original stand in opposition to the arms race today and voted to call for a "halt" instead of "curb" to the testing, production and deployment of new nuclear weapons in their forthcoming pastoral letter on war and peace.

The vote was a defeat for the Reagan administration, which had sought to soften the language of the controversial letter. Some bishops read it also as an endorsement of a nuclear freeze.

On Sunday Archbishop John R. Roach of St. Paul-Minneapolis, president of the National Conference of Catholic Bishops, told reporters that in his view "halt" and "freeze" mean the same.

The bishops' five-member drafting committee previously had avoided any reference to a "freeze" to avoid entanglement in the freeze debate. In House debate on a freeze resolution in recent weeks both sides have claimed that the bishops' letter favors their position.

About 275 bishops have gathered here to vote on nearly 500 proposed amendments to the 150-page third draft. The letter they produce will be the official position of the Roman Catholic church in the United States on questions of nuclear arms and nuclear war.

The drafting committee originally used the word "halt" in the letter, then in what was generally viewed as a softening amendment switched in the third draft to "curb."

Last week it went back to "halt."

That was partly in response to those who felt the third draft had become too soft. Administration officials who criticized the second draft praised the third. Roach and Cardinal Joseph Bernardin, chairman of the drafting committee, then issued a joint statement saying "any suggestion that there are relatively few and insignificant differences between U.S. policies and the policies advocated in the pastoral" is wrong.

As Bishop James Malone of Youngstown, Ohio, one sponsor of the halt amendment, said today, "Halt sets the tone."

There was no count after the standing vote on the amendment, but it was clear that it had the two-thirds necessary to pass.

In another shift back to earlier wording, the bishops approved a substitution stating that they "oppose" rather than "resist" the addition of first-strike weapons and strategic nuclear planning that "goes beyond the limited function of deterrence." The document accepts nuclear deterrence only as a step toward disarmament.

An attempt by conservatives to delete two pages on nuclear deterrence, including the passages on the halt to the development of new nuclear weapons and the opposition to first-strike weapons, was defeated.

Though the vote on the halt amendment indicates that the letter is likely to be adopted--the schedule calls for a final vote Tuesday--the proposed amendments span the political spectrum.

Bernardin said in remarks to the bishops that "on such a complex set of questions as we have addressed, we do not expect unanimity of each specific judgment." But, he said, "consensus" exists in the church on the "moral principles of warfare."

Bernardin declined to be drawn into whether the word "halt" implied stopping weapons programs now underway. "We are not spelling out the concrete implications," he said.

The document opposes nuclear war and calls for major arms reductions by the Soviet Union and United States.

It rules out deliberate nuclear attack on civilian populations even in retaliation for an attack on an American city, and condemns first use of nuclear weapons. The draft's "conditional moral acceptance" of possession of nuclear arms as a deterrent to war is applied specifically to the problems of defense of western Europe.

An attempt by Auxiliary Bishop Patrick V. Ahern of New York to postpone votes on the proposed amendments and a final vote on the document until November was voted down today.

The large number of resolutions also prompted an informal study session Sunday night involving about 25 bishops, many of them members of the Catholic peace movement Pax Christi.

The proposed amendment to return to "halt" was submitted by 11 bishops, one of whom was acting on behalf of 13 Texas bishops. Six other proposed amendments suggested replacing "curb" with "cease," "freeze" or "stop."