Why, Cardinal Joseph Bernardin was asked, had the bishops retreated from "halt" to "curb" in a crucial passage about the Reagan arms buildup in the third draft of their pastoral letter on war and peace?

Last November, in their second draft, they had called for a halt in the production, deployment and testing of new nuclear weapons. This week, it was the more equivocal "curb," a word that has been seized upon by liberals as evidence that the bishops had lost their nerve--or at least their opportunity decisively to influence the outcome of next week's House debate on the nuclear freeze.

"It's a long story," sighed the cardinal, the gentle, buffeted chairman of the drafting committee.

The pressures from the White House, where the bishops' efforts to make nuclear warfare a moral question are regarded as ecclesiastical effrontery, have been visible and heavy. In November, President Reagan's national security affairs adviser, William P. Clark, sent a long letter to the bishops' conference, in which he angrily accused them of misreading the Reagan position on arms control and misleading the public.

Then, the mood was of cheerful defiance. The bishops seemed to be headed toward confrontation with the "Peace Through Strength" doctrine of the administration. They seemed about to endorse the freeze, condemn first use of nuclear weapons and challenge the morality of the "deterrence" theory, which justifies the possession of an enormous nuclear arsenal.

After studying the Clark manifesto, Cardinal Bernardin calmly told a news conference, "We will see who is misleading whom."

Tuesday, at a meeting of the National Catholic Education Conference, where the cardinal explained the new draft, it was all quite different.

"We were very sensitive to White House comments," he said as he fended off charges that the document was a backdown.

It is no secret that one member of the drafting committee, Bishop John J. O'Connor, former chief of chaplains of the U.S. Navy with the rank of rear admiral, vigorously represented the Pentagon point of view.

It was at his insistence that "halt" was reduced to "curb." He pressed for a vote on it, and won. He also lobbied hard to delete all references to "first use." He lost, but the passage that appears allows for individual interpretation by men of good will--a latitude not afforded in church pronouncements on birth control or abortion or other matters relating to the private lives of Roman Catholics.

"Bishop O'Connor came to every meeting with 40 pages of suggested changes," said one disgruntled priest.

Sensitive as they may have been to White House disapproval, the drafters may have been even more so to pressure from Rome, and a militantly anti-communist Polish pontiff. Pope John Paul II summoned Bernardin and another member of the drafting committee, Archbishop John Roach of Minneapolis, to Rome in January, to "consult" with bishops from Germany and France, who were in a state of panic and outrage at the mad course being pursued by the Americans.

"It was all very brotherly," one observer said ruefully. "But there was no mistaking the drift."

A report on the consultation, entitled "A Vatican Synthesis," contains circumlocutions like this: "Is it preferable to address the practical choices in more nuanced and tentative ways using hypotheses and indicating the limiting conditions?"

This translates to "no specifics." Bishop O'Connor is said to have used the letter from Rome to good purpose within the committee.

The pope is against nuclear war and all that, but he has institutional concerns. He did not want a document that would split the church.

The fears of the French and German hierarchies are echoed in American parishes, although 82 percent of American Catholics polled said they favor a freeze. Writing in Rolling Stone, William Greider reports the firestorm of opposition encountered by Bishop Walter Sullivan of Richmond as he tried to explain to his flock "the gross evil of nuclear destruction," which, he said, "obliterates the traditional--indeed church-condoned--rationale for military conflict."

One of the congregation yelled "Russian propaganda" at him; another said that Catholics look to bishops for "our moral values, not for our national security."

Those elements within the church willbe appeased by the "nuanced" third draft.But it could open up a division on theleft.

About half of the 300 U.S. bishops have endorsed a freeze and might try to write it back into the draft on which the entire U.S. conference will vote in Chicago next month. But it would be too late for the hotly contested vote in the House. The White House has expressed satisfaction privately with the "more flexible" version that has so disappointed those who had been hoping for a moral thunderclap.