Leaders and participants at a B'nai B'rith seminar here yesterday were almost universally critical of President Reagan's linking of religion and politics in a speech at the Republican National Convention last month.

Reagan's statement that religion and politics are "necessarily related" was "almost word for word the kind of statements that the Ayatollah Khomeini has made" in Iran, said Barry Rubin, of the Georgetown University Center for Strategic and International Studies.

Rubin, a Middle East specialist, and others addressed a workshop at the five-day international convention of B'nai B'rith.

The speakers at the seminar titled, "Is the Growing Role of Religious Movements in Shaping Public Policy a Threat to Democracy?" sought to define what is acceptable involvement of religious groups in the political arena.

"It is perfectly valid for people of a religious persuasion to put forth their ideas" in efforts to influence public policy, said Eugene J. Fisher, who heads Catholic-Jewish relations for the National Conference of Catholic Bishops.

"But it must be done in ways that can be debatable in the public forum," he cautioned. "There's a tremendous danger when any religious group decides only its way is God's way."

Rabbi David Saperstein, director of the Religious Action Center here for Reform Judaism, warned that "we are seeing people trying to impose their religion on America."

Saperstein agreed with Fisher that "religious groups can speak out on issues." But he criticized sharply the actions of "someone like the Rev. Jerry Falwell to come along and say, 'God has told me that you ought to vote for candidate B.' "

The TV evangelist and Moral Majority leader has been a close adviser to Reagan on religious issues and played a key role at the GOP convention in Dallas last month.

Religious groups "can speak out on issues," Saperstein said, "but they cannot spend one penny" on partisan political activity.

Saperstein condemned sharply Nevada Sen. Paul Laxalt's "Dear Christian Leader" letter, sent to Texas clergy just prior to the convention, urging voter registration drives in churches to "help secure the reelection of President Reagan and Vice President Bush."

Fisher warned of the dangers, both to religion and politics, "of reducing religious values to one or two issues."

In response to a question about Reagan's advocacy of religious exercises in public school because the nation has become "Godless," Saperstein denied the charge. Studies have shown that "religion plays a major role in our lives . . . .

"In any case, you don't secure religious values in our lives by trying to legislate them."

He challenged the president to "show that America is unique because of its religious pluralism" instead of branding as "anti-religious" those who speak out for separation of church and state. "What the president does is constitutionally illegal," Saperstein charged.

Both Mondale and Reagan are scheduled to address the 1,200 delegates to the Jewish service organization on Thursday.

Other issues to be addressed at the convention at the Sheraton-Washington Hotel this week include anti-Semitism, black-Jewish conflict, relations between Israel and Jews in other parts of the world and Jewish survival.

An internal issue expected to generate some conflict is a proposal to give women "full and equal membership" in B'nai B'rith.