A Northern Virginia woman who says rock 'n' roll music can hook teen-agers on sex and drugs and who calls welfare payments part of an international Communist plot played a major role in persuading the National Park Service to include a nativity scene in the government's Christmas pageant on the Ellipse.

Anne Neamon, founder and national coordinator of a McLean group called Citizens For God and Country, yesterday described the Park Service decision as "an unbelievable achievement" and one that will help "secure the moral order" of the world.

The decision to include the scene as part of the service's annual Christmas Pageant of Peace was disclosed Tuesday and immediately denounced by the American Civil Liberties Union as a violation of the separation of church and state. "It looks like it's the government favoring one religion or condoning one religion," ACLU attorney Elizabeth Symonds said yesterday.

Neamon, who is best known in Northern Virginia for her challenges to expanded sex education classes in Fairfax County, was ecstatic about the Park Service action and furious at the ACLU's continued opposition to the display. The ACLU, she said in a telephone interview, is "a subversive organization," committed to toppling the established order in America.

Neamon remained vague about her own background and her organization, saying she could not reveal the group's address, her home address, or her age for fear of retaliation by Communists. She also disavowed a 1980 interview in which she was quoted as saying that she was the organization's only member. And she declined to discuss the size of her group, which lists a McLean post office box as its address. "Just indicate that for security reasons, I am not responding," she said.

By inundating the Park Service with scores of letters, each topped by her organization's logo -- a cross, the Bible and the American flag -- Neamon proved to be "by far" the most vocal advocate of including the nativity scene, said Park Service spokeswoman Sandra Alley. "She is very persistent," Alley said. "She is definitely a strong proponent of what she believes in."

Alley said Neamon had been lobbying the service for 11 years, arguing that the 1973 court order requiring the scene to be excluded from the pageant was wrong. It was not until March, when the Supreme Court ruled in a Rhode Island case that such a display can be placed on public land without violating the Constitutional separation of church and state, that the Park Service began to listen, said the spokeswoman.

The service called a meeting on the issue Nov. 1, and among those present was Neamon. Park Service lawyers said that under the Supreme Court ruling there was nothing improper in including the nativity scene and Alley said the service agreed to include it in the pageant, which draws thousands of visitors to the park, south of the White House.

The only problem, she told United Press International, is that the government doesn't own a nativity scene. A Vienna man has offered to donate one, Alley said, and the service is considering his offer.

Neamon said the display will help restore the Christian ethic she says this country was founded on. It won't, however, answer all her problems with the government pageant.

She cited her worries over the National Christmas Tree, also part of the pageant. "They have been decorating the tree with the six-pointed star, which is the Star of David, instead of the five-pointed star, which is the Christian star," she said.

Neamon said she is also troubled "that the program is referred to as a 'Pageant of Peace,' which is a Soviet term."

The decoration atop the tree changes every year, Alley said. This year the ornament is a three-foot replica of a white poinsettia.

Neamon expressed delight at the nativity scene decision. "This should be a good lesson to those who take a despondent attitude and say: 'What can I do -- one person?"

There are plenty of issues left for her group, she said. They include school prayer ("It belongs in the schools because we are one nation under God"), abortion ("If you make an exception for rape, then every woman will say 'I've been raped' "), gay rights ("The plot is to destroy one generation of youth"), rock 'n' roll music ("It is illegal, it's just not being enforced"), and sex education, which is "hostile to the Christian ethic . . . . "and whenever there is any religious straying and betraying, there is always trouble, trouble, trouble."

When educators in Fairfax County in 1977 considered expanding their sex education program, Neamon was there. "She would call me up and read from the Bible," said Rodney Page, then chairman of the county school board.

That was a battle she lost. The county expanded its sex education program despite her objections.