When Gerald B. Leib wrote to the Education Department to complain about an official distributing a speech that called America a "Christian nation," he was startled at the reply.

Christopher C. Sundseth, a Reagan administration appointee at the Treasury Department, got hold of Leib's postcard and fired off a stinging response, calling the Mountain View, Calif., resident an "amazing, pathetic creature."

Sundseth, 31, said he believes that he obtained the card from one of four Christian activists who, he said, regularly file Freedom of Information Act requests with federal agencies for correspondence dealing with Christian issues. He said these friends, whom he would not identify, send such letters to him and that he often writes to the senders.

"We are indeed, like it or not, a 'Christian nation' as more than 85 percent of adult Americans consider themselves 'Christians,' " Sundseth said in the letter to Leib, which Sundseth mailed from his Alexandria home. "This country was founded by Christians who were escaping the same kind of small-minded tripe you espouse.

"The framers of the Constitution attempted specifically to anticipate those of your ilk who would try and abridge the very rights of freedom to worship guaranteed us by that document . . . . You are a truly amazing, pathetic creature."

Sundseth added: "P.S. When you die, you will be giving account to Jesus Christ, your creator, who happens himself to be a Christian. I hope you are prepared . . . . "

Sundseth, a GS-13 special assistant detailed to the Inter-American Development Bank, said he saw nothing wrong with calling the United States a Christian nation.

"To Christians, it's as controversial as saying it's sunny outside," Sundseth said. "It's a fact. The antireligious, hostile people find it controversial.

"In my free time, I'm a Christian activist," he said. "I write a lot of letters like that . . . . I didn't give up my right to express my opinion when I came to government."

Sundseth, who is a former director of the Adolph Coors Co.'s political action committee and a fund-raiser in Colorado for Presi- dent Reagan's 1980 campaign, received his political appointment last year.

His mother, Carolyn B. Sundseth, is a White House public liaison officer who deals with evangelical and fundamentalist Christians.

Sundseth said his activist friends use the Freedom of Information Act "to find letters of antireligious zealots . . . . Anybody that says anything about Christians is automatically filed by these people. It's a knee-jerk reaction. It's no big conspiracy. It's a couple of Christian guys writing letters."

Sundseth said that "it's a biblical injunction to warn people" about eventually facing Jesus Christ, "so I warned that guy. I'm saying nothing different than what is in the Bible."

Sundseth said he believes that Leib's card and many similar letters are orchestrated by People for the American Way, a liberal group founded by television producer Norman Lear. A spokesman for the group said that Leib is a member and that the group called for the Education official's resignation over the Christian-nation speech but did not urge anyone to write the Education Department about the issue.

Leib, who never received a reply from the department, complained to Rep. Patricia Schroeder (D-Colo.) about Sundseth's "gratuitous proselytizing" and questioned who Sundseth is. Schroeder has asked the Education Department how Sundseth obtained a copy of Leib's March 19 card so quickly that he replied April 3.

Leib's card was addressed to Tom Tancredo, the Education Department's representative in Denver and an acquaintance of Sundseth. It followed a well-publicized incident in which Tancredo mailed to Christian schools in his area a speech saying godlessness had taken over "this Christian nation." The speech had been written five years earlier by former Moral Majority leader Robert J. Billings, then an Education Department official.

Department official George D. Youstra told Schroeder that Leib's card apparently had been obtained via Freedom of Information and that he regretted "the unpleasant letter" from Sundseth. But he said Tancredo, who did not return phone calls yesterday, could not control how others use such documents.

Education Department spokesman Lou Mathis said that, because of the incident, the department will no longer release names and addresses of private citizens writing unsolicited letters.