American religious publishers, many of them already in perilous financial condition, are banding together to fight a proposed federal budget cut that they say would force a large number of religious newspapers and magazines to cease publication.

Among many other cost-cutting measures, the Reagan administration is seeking to eliminate a $289 million postal subsidy to nonprofit second-and third-class mail users, a subsidy regarded as the lifeblood of U.S. religious periodicals.

The prosopal is subject to congressional budget review, which will continue for the next few months.

Publishers, who have been battered by sharply escalating distribution costs, say that the proposes cut, if approved, would double postage bills for the typical religious journal on Oct. 1 on this year.

"It would threaten the existence of an independent noncommercial press in this country," warned the Rev. Donald Hetzler, executive secretary of Associated Church Press, which represents about 135 Christian magazines and papers from its headquarters in Geneva, Ill. "There's no doubt a large number of our member publications would go under."

"It's a dastardly deed on the administration's part," said Alan Caplan, the Washington liaison for the American Jewish Press Association.

The proposes postal cut is one of a number of items American religious leaders are complaining about in response to the administration's economic program.

The general secretary of the U.S. Catholic Conference, Bishop Thomas C. Kelly, has charged that the budget-trimming plans effectively abrogate the government's "moral responsibility" to care for the poor.

Several key religious bodies have expressed strong disapproval of the White House's attempt to slash cultural and social programs apparently in order to pay for a $220 billion military budget.

The postal proposal, however, would have a direct impact on religious organizations -- and on the newsletters and journals of many other charitable and special-interest groups -- by ending a congressionally approved 16-year plan to eliminate gradually certain mailing privileges for nonprofit users. The plan has six years left.

The cut is being opposed formally by a hastily formed coalition of nonprofit organizations and publishers, including such groups as the March of Dimes, the National Audubon Society, the National Conference of Catholic Charities and the nation's four major religious press associations.

Under the proposed plan, an attorney for the coalition said, annual postage costs for the National Catholic Development Conference would increase by $18 million, for example, and the Billy Graham Evangelistic Association would face an additional $2.6 million in mailing costs each year.

The full force of the cuts probably would be felt most keenly by religious publications, analysts say, especially those with limited access to advertising revenue and denominational subsidies.

"This has the dimensions of disaster for us," said James A. Doyle, executive director of the Catholic Press Association in Rockville Centre, N.Y., the country's largest religious press grouping with 300 members publications. "The voices we represent will be mutted, and in some cases silenced. And that's not what government is supposed to do."

Doyle said the rapidly increasing postal costs over the last decade have altered the face of religious publishing in America substantially, forcing scores of publishers to reduce the number of issues they produce each year.

"The new cut," said Doyle, "would bring us to a position where we can no longer economize. The money just isn't there any longer."

The Rev. John Stapert, editor of the Reformed Church in America's Church Herald magazine in Grand Rapids, Mich., and postal affairs liaison for both the Evangelical Press Association and Associated Church Press, said that many Protestant publications already have begun to lose touch with their readers because of economic retrenchment.

Stapert's own magazine now publishes 26 issues a year, down from 45 in 1974. Still, the mailing costs for the Church Herald have nearly tripled during those years.

"We now spend more on postage than on salaries," said Stapert.