Federal investigators have shed new light on how a Christian activist at the Treasury Department obtained a citizen's post card to the Education Department, enabling the official to fire off a stinging reply.

Christopher C. Sundseth, a political appointee at Treasury, initially told department investigators that he could not remember how he obtained the post card sent to the Education Department's Denver office, according to sources familiar with the probe. Sundseth told The Washington Post last month that he believes 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.

But in a second interview under oath with aides to Treasury's inspector general, sources said, Sundseth acknowledged that he obtained the post card last March after calling the Education Department's Denver office, not through a FOIA request. Sundseth said he had wanted to protect the official he called in Denver, according to sources.

Sundseth also said he does not recall whether he wrote the reply in his Treasury office or at home, the sources said. But he said he sometimes writes private letters on a personal computer in his office and sees nothing wrong with this because he works 50 to 60 hours a week and often takes work home.

A Treasury official confirmed Sundseth's call to Denver in a letter to Rep. Patricia Schroeder (D-Colo.), chairman of the House civil service subcommittee, which is investigating. The letter said the probe "did not disclose any evidence that Mr. Sundseth used his government position to obtain these documents or that he used government resources inappropriately."

Treasury Inspector General John C. Layton has suspended his inquiry until the Education Department's inspector general finishes reviewing the matter, the letter said.

The episode began when Gerald B. Leib, a Mountain View, Calif., attorney, sent a post card last March 19 to Tom Tancredo, the Education Department's representative in Denver and an acquaintance of Sundseth. Leib complained about a speech in which former department official Robert J. Billings had called America a "Christian nation."

Leib was startled to receive an April 3 reply from Sundseth with only his Alexandria home return address. The reply called Leib an "amazing, pathetic creature."

Sundseth could not be reached yesterday. He said last month that in his free time "I write a lot of letters like that" to "antireligious zealots . . . . I didn't give up my right to express my opinion when I came to government."

A further complicating factor involves George D. Youstra, Tancredo's supervisor, who initially responded to Schroeder's inquiries. Youstra's wife works for Sundseth's mother, Carolyn, who is the White House liaison assigned to deal with fundamentalist Christians.

Leib said yesterday that he "thought it was kind of bizarre" when Sundseth answered his post card. He said he recognized the last name and recalled he had written to Carolyn Sundseth last year after she said in a speech that aides to President Reagan should "get saved or get out" of government.