Last Sunday evening at the First Baptist Church of Washington, which is President Carter's house of worship, the senior minister, the Rev. Charles Trentham, delivered an eloquent sermon on the evils of militarism.

"Nobody wins a war," said Trentham. He referred to Christ's command to "put up your sword." He said the sentiment of Pope Paul's cry at the United Nations, "No more war," was an absolute necessity in these days of a global arms race. In calling on the congregation to become active against the threat of war, he asked it to be peaceful and stop hating.

As part of a weekend "peace convocation" that brought a number of peace movement strategists to First Baptist - William Sloane Coffin, Cora Weiss, Richard Barnet, Marcus Raskin - the sermon illustrated both the frustrations and opportunities that many churchmen face. It is praise-worthy for Trentham to get himself and his church of middle- and upper-class believers thinking about the arms race and the nuclear threat, and assuredly the melody of his sermon hit all the proper notes.

But is that enough?

The war lobby - the congressional committees that keep the billions flowing to the Pentagon, the defense contractors that build the weapons, the theorists who keep insisting that it will be the sinister Russians - not us - who blow up the world, the boosters for the revival of the draft - isn't at all bothered by convocations against militarism. It welcomes them.

The war lobby enjoys these discussions, because, after so many years of them, it knows they are useful fronts. So long as religious leaders tamely think that progress toward peace comes about from convocations, seminars and dreams of creating a National Peace Academy, or that this dialogue represents a meaningful alternative to America's militarism, then there is little to worry about.

What is needed from churchmen like Trentham is a little less eloquence and a lot more grit. An opportunity exists to instruct the faithful in specific ways to test the system.

One of the most effective means for confronting the military is for citizens to stop paying for the machinery of war. Small peace groups, such as the War Resisters League and the Center on Law and Pacifism in Philadelphia, have been going down this path, but until the churches of the vast middle way begin to spread the word about the specifics of military tax refusal, the defiance can be dismissed as the protests of the anarchistic few: It's unpatriotic, it's stupid and it won't work. Worse, it will make use vulnerable to our enemies who thrive on signs that we are going "soft."

Actually, the softness to be feared is the squishy thinking that can't see the link between flourishing militarism and the payment of taxes that support it.

Citizens are right to be anxious about tax resistance. Visions of the IRS slapping one into prison are frightening enough for the individual. But if he or she is a job-holder with a family to feed and care for, a social position to keep and is perhaps already involved in some form of altruism, then taking on the state via tax resistance may be too much to think of.

The role of the church leaders would be to reduce the risks of defiance by increasing the number of defiers. If enough citizens, for reasons of conscience, refuse financial complicity with the military machine, the state has a choice of two responses: arrest every last one of the brazen or get out of the war and weapons business.

If the latter occurs, national security would then mean the security that a nation enjoys when its citizens are adequately fed, housed, educated and cared for. At the moment, this country has the greatest arsenal of weapons in history, yet before us remains the specter of national insecurity: inflation, unemployment, fears of a fuel shortage.

At the moment, policy makers have immense control not only for what military schemes the public's money may be used or squandered, but also in influencing the thoughts of citizens who would prefer not to cooperate with the process. Who really knows what constitutes legitimate defense? The religious leaders of the community - those who sponsor convocations that pray for peace but say nothing against paying for arms - represent one of the few institutions that can counter the controls of Pharoah. If they won't lead, who will?

Even those who are further out front than most keep missing opportunities. Last Sunday morning, William Coffin had President Carter in the congregation at First Baptist. Instead of using the moment to talk of meaningful action like tax resistance - Coffin himself, to his credit, is withholding his war taxes this year - he thundered still again about "America's obsession with military power."

After the services, Carter told Coffin "that was a superb sermon." The illusion was sustained. Carter could still see himself as a peacemaker, even though he increases the military budget, regularly gives approval to new weapons of dubious worth and secures Middle East "peace" by putting up $5 billion of America's money for new military equipment for Israel and Egypt.

Coffin could have given a truly memorable sermon merely by sharing his thoughts about his personal commitment to tax resistance. That would have been new material. A strong oration calling for civil disobedience as a way to counter "the military obsession" would likely have drawn a different response from Carter.

As it was, warm praise from the military's commander-in-chief must have made Coffin wonder what he said that was wrong or tepid.

Little of this would matter, execpt that the sponsors of the convocation at First Baptist sought attention for what they were doing. It is true that Trentham could have stayed clear of the project, which is what the majority of his fellows do.

It is known, also, that many in First Baptist are mildly enraged - and some more than mildly - by their pastor's sponsorship of last weekend's events and discussions. Baptists giving respectable platforms to a Coffin or Raskin is seen by this faction less as a drift to the left than as a heretical lunge.

In the context of American religion, the convocation had a measure of boldness. A comfortable parish was nudged a bit to think about war and peace. But in the larger context, where millions of victims of global militarism exist in misery, nothing changed. It was another instance of moral leadership not daring to step beyond the safety of moralizing. CAPTION: Picture 1, THE REV. WILLIAM SLOANE COFFIN . . . withholds war taxes; Picture 2, DR. CHARLES TRENTHAM . . . "Nobody wins a war."