Washington — Walter Mondale, as he sometimes says, is a ''preacher's kid.'' His dad was the Rev. Theodore Mondale, a Methodist minister in rural Minnesota. Walter's mother, Claribel, was a director of religious education. In 1955, Walter married Joan Adams, the daughter of a Presbyterian minister.
Presidential candidate Mondale brought that kind of rich religious background with him when he stepped before the international convention of B'nai B'rith here on Thursday. His topic: the role of religion in the 1984 election - including the role of the religious right in the campaign of the Republicans and President Reagan.
Mondale, speaking in quiet tones, warned of a ''moral McCarthyism'' arising in the land. He asked why some fundamentalist Christians are branding him ''antifamily and un-Christian.'' He cautioned that some religionists are seeking to ''impose their own beliefs on other people.''
Aides said that the speech reflected Mondale's ''growing anger'' over attempts by some foes to paint him and the Democratic Party as unpatriotic, immoral, and unreligious. He also accused the President of exploiting religious zealotry, rather than using his office to encourage religious pluralism.
''What I am doing here today is something that ... I never thought I would do: I have never before had to defend my religious faith in a political campaign.''
Mondale said he was ''alarmed'' by what he had heard from the President and the religious right. ''Most Americans would be surprised to learn that God is a Republican,'' he quipped.
In Mondale's boyhood home liquor and tobacco were forbidden and lying was punished by the vigorous application of a switch. In his speech here, Mondale used a verbal switch on the Republicans.
The President, he charged, has crossed an important line that is vital to religious freedom in the United States. He observed that Mr. Reagan told a prayer breakfast in Dallas last month that religion needs defenders against those who care only for the interests of the state. Mondale continued: ''His clear implication was that he welcomed such a role for himself and suggested that we were threatened by leaders with no interest in the religious life of our country.
''Well, the Queen of England, where state religion is established, is called Defender of the Faith. But the president of the United States is the defender of the Constitution - which defends all faiths.''
Of particular concern, said Mondale, has been a tendency by Republicans to break down the ancient wall between religion and government in America. He asserted: ''Mr. Reagan attacked those of us who are trying to preserve those constitutional principles, the separation of church and state. He supports a constitutional amendment instituting school prayer, with the prayers chosen by local politicians. In Dallas, he said that anyone who opposes that amendment is 'intolerant' of religion.
''Tuesday, in Utah, Mr. Reagan attacked again, accusing us of favoring 'freedom against religion.' Instead of construing dissent from him in good faith , Mr. Reagan has insulted the motives of those who disagree with him - including me.''
Mondale attempted in this speech, which he had been carefully considering for days, to set forth his own beliefs on the role of religion and government in America. This is how he described his own positions:
''I believe in an America that gives, as George Washington wrote to the Touro Synagogue, 'to bigotry no sanction, to persecution no assistance.'
''I believe in an America that has been a home and a refuge for people of every faith. Our government is the protector of every faith because it is the exclusive property of none.
''I believe in an America that honors what Thomas Jefferson first called the 'wall of separation between church and state.' That freedom has made our faith unadulterated and unintimidated. It has made Americans the most religious people on earth. Today, the religion clauses of the First Amendment do not need to be fixed, they need to be followed.
''I believe in an America where government is not permitted to dictate the religious life of our people; where religion is a private matter between individuals and their God, between families and their churches and synagogues, with no room for politicians in between. I do not for one moment claim a partisan monopoly on the beliefs that I've just outlined. They are the common heritage of all Americans: Protestants, Catholics, and Jews; Democrats and Republicans - everyone....
''To coerce is to doubt the sturdiness of our faith. To ask the state to enforce the religious life of our people is to betray a telling cynicism about the American people.''
Reagan staffers say they would like to see the church-state issue put to rest. Mondale aides say that won't happen. Mondale himself suggested that the President is trying to ''slip out the back door'' on this one, and that he may be worried that the whole issue could backfire.
Mondale showed he wasn't about to let the issue die. After addressing B'nai B'rith, he took the same message a few blocks away to deliver it to some 10,000 people attending the National Baptist Convention.