IT IS ROSH HASHANAH, the Jewish New Year. The scene is a middle-class. Brookline synagogue, but it probably could be one of any number of temples around the United States. The voice from the pulpit is determined, and as the speech continues, it becomes deeply passionate and, at times, strident.

They say a picture is worth a thousand words, the rabbi asserts rather subduedly. But starting this summer, he adds bitterly, a picture taken in Lebanon gave a thousand lies! And with that, he launches into a caustic denunciation of what he perceives as the media's extreme bias against Israel, listing numerous cases where journalists unhesitatingly bought the Arab line. Speaking without notes, this youngish, bespectacled man slips from attacking the press to a general tirade defending the invasion of Lebanon.

All the predictable themes come spilling out: why it was regrettable, but necessary; how the death of even one innocent civilian is a catastrophe; how Israeli soldiers went to great personal risks to protect civilian life; what a double standard the world judges Israel by. There is no applause when the rabbi finishes: this is a synagogue. But the audience plainly approves, for their feelings about the issue have been eloquently expressed.

The rabbi's interpretation of events was entirely reasonable, if open to debate. But what was striking about the speech was not the substance, but the tone, the disturbingly complete lack of even questioning Israeli policy and tactics. It's not that the rabbi was merely passionately defending Israeli. It's that he would not even consider anything less than total, down-the-line support for every one of its actions.

American Jews faced trying times this past summer in the wake of unpopular Israeli military excursions. Though their support for their brethren remains rock-ribbed in places like Brookline, many are deeply unsettled about the Begin-Sharon defense plan and what it bodes. These fears must not be swept under the carpet.

More than anyone else, American Jews can serve as an effective conscience for the Israeli government. Forget some critics' contention that U.S. Jews have no right to speak out because they are not living under Israel's constant stress and turmoil. Their dreams and aspirations are bound up with Israel's, and to shut out their voices smacks of the worst sort of self-righteous censorship. Especially when they are voices of dissent.

The issue of Israel's existence or survival is not in doubt anymore; more than 30 years of fortitude in response to Arab rejection has assured that. But Sharon and Begin's road is not necessarily the only one to peace. American Jews must feel free to differ from the party line.

They are, after all, credible critics for the most part. When Thomas Dine, executive director of the vociferously pro-Israel American Israel Public Affairs Committee, last week praised part of President Reagan's Middle East initiative bitterly condemned by Begin, you knew he wasn't out to gut the Jewish state.

The worst thing American Jews can do is blindly accept all Israel's positions pro forma, as rabbis all around the country probably did last weekend. That can only harden Israeli intransigence. American Jews have to abandon their traditional practice of rallying to the government line just because it has few other friends. Israel is a strong, tough state, in no need of coddling. Only by being honest with their feelings and speaking out can American Jews do their most for the Jewish state. Even if that honesty and speaking out means a little bit of friendly criticism.