THE HUGE demonstration in Bonn over the weekend was an event on which Americans need to reflect carefully. Some of the music was American, and German students look very much like their American counterparts. But it would be very foolish to dismiss this outpouring as a mere echo of the peace movement in this country. The Bonn meeting drew on sources of strength that are specifically German and that appear to be increasing.

Chancellor Helmut Schmidt is committed to NATO's deployment of new nuclear missiles in Europe, but substantial elements of his own party, among the left wing and the young, are vehemently challenging him. Although the substance of American policy in Europe has changed little in the past couple of years, the arrival of the Reagan administration and the rhetoric of the American right-- particularly the talk about nuclear superiority-- have evoked a vigorous reaction on the German left.

But that reaction is not confined to the traditional Socialist left. There's also been a powerful revival of radical turn-the-other-cheek pacifism in Germany's Protestant churches. There was an astonishingly large turnout for the Protestant assembly in Hamburg last June, with tens of thousands of young people in sweaters and jeans among the customary attendance of pastors and philosophy professors in their dark suits. The central subject was nuclear weapons, and the central purpose was to protest their deployment. To the German government's dismay, the Protestant positions on the weapons issues tend to be expressed in the moral absolutes of academic theology. It was, incidentally, the religious pacifists, not the politicians, who organized the demonstration in Bonn Saturday.

Chancellor Schmidt has always answered this kind of protest by pointing out that German diplomacy over the years has played an important and productive part in the strategy of the Westeran alliance, encouraging continued efforts at arms negotiations with the Soviets. The United States helps Mr. Schmidt, and strengthens the political base of NATO in Germany, when it proceeds actively with negotiations. Mr. Reagan, like every American president, has discovered that in military and foreign policy his constituency is not limited to American voters.