John Steinbruck once considered himself as something of a military hawk. Back in 1949 he enlisted in the Navy. Years later he returned to the service as a Protestant chaplain, still subscribing wholeheartedly to the motto: ''Peace through preparedness.''
But on Memorial Day, this Navy reserve chaplain found himself on the side of those speaking out against the direction of US arms policies.
''Previously you could develop a rationale to justify certain wars,'' he said , standing amid the throngs of American religious groups attending anti-nuclear rallies in front of the White House. ''But today's nuclear weapons simply do not discriminate between soldiers and innocents; (they) are designed for total massive destruction and could wreak unimaginable damage to the environment. Now, if honest, I simply cannot think of any conceivable circumstances under which I could justify nuclear warfare.''
John Steinbruck, with his anti-nuclear conversion, is typical of the thousands of religionists who staged anti-nuclear peace rallies across the nation May 30 and 31.
Ostensibly these gatherings did not have specific political goals. Most involved a combination of speakers, prayer, and common reflection on reversing the nuclear arms race--a kind of apolitical pep rally for the special United Nations session on disarmament that begins June 7.
But if these gatherings are any indication, the ''just war theory''--under which military action is judged in some instances to be morally justified--is now losing its grip on a wide cross-section of Christians and Jews. That could translate into new support for anti-nuclear political forces in the coming months.
Participating churches and synagogues this weekend, anxious to register their growing concern, held meetings near nuclear weapons installations and manufacturing plants across the country, including missile sites at Rapid City, S.D., the Savannah River nuclear weapons facility in Aiken, S.C., the Honeywell Company's plant in Minneapolis, which makes components for the MX and Minuteman missiles, and the General Electric plant at Philadelphia, which produces the Mark 12A warhead.
Other interdenominational meetings were staged in major cities such as Sacramento, Calif., Houston, Wilmington, Del., and Washington, D.C.
In recent months, well-known theologians and the faculties of major theological seminaries also have been openly critical of the ''just war theory.'' Evangelist Billy Graham, reversing a former stand in support of nuclear armaments, now puts the disarmament issue at the center of his ministry.
The Harvard Divinity School went on record recently: ''While there may have been just wars in the past, the inability to place traditional constraints on nuclear war now makes any moral justification impossible.'' A similar position has been taken by Union Theological Seminary in New York and the Pacific School of Religion in Berkeley.
Among the religionists now speaking out, four main concerns seem to be feeding into their assault on the ''just war theory'':
* Many are convinced that America's new efforts to bolster its nuclear arsenal will actually destabilize the current nuclear balance and runs totally counter to the peacemaking demands of their religious traditions.''
We're concerned that mutual deterrence is no longer the operating strategy of the United States,'' says the Rev. Currie Burris, coordinator of the disarmament program of Clergy and Laity Concerned in New York. The present US policy is ''to achieve first-strike capability,'' he says.
* Concerned Roman Catholics link their concern to the church's anti-abortion position. ''Our central principle has been that you cannot take innocent life,'' says Rev. Bryan Hehir, director of the Office of International Affairs for the US Catholic Conference. ''And that same moral principle guides our thinking in the nuclear area--where weapons cannot help but take innocent life.''
* For others, the reliance on nuclear weapons for national security has begun to loom as the supreme idolatry of this age.''
When weapons become the basis of our security, they become our idols,'' says Joseph Lynch, of the Sojourners evangelical Christian community in Washington, D.C., that publishes a national magazine on religion and social issues. ''We lose dependence on God.''
Sojourners, which coordinated demonstrations nationwide, is trying to make disarmament the abolitionist movement of the 20th century, as slavery was in the 19th.
* Those of more liberal political persuasion increasingly see the vast defense budget as an unacceptable drain on the nation's efforts to help the poor and needy.