SONIA JOHNSON is 43, the wife of a professor, a mother of four, who lives in Sterling, Va. She is also a fifth generation Mormon who is violating the church's teachings by leading an organization called Mormons for ERA. It has not, she says, been easy.

Mrs. Johnson says the Mormon church is isolating itself from the mainstream of American life by insisting that "the only legitimate occupations [for women] are motherhood and wifehood." Several years ago, she says, she heard her minister read a letter from the head of the church telling her congregation that "ERA was immoral, that it would destroy the family, that members of the church should fight against it. The minute I heard that letter I just said to myself this is not true, I cannot go along with that."

Since then, she says, she and other Mormon women have found themselves at odds with the church, ostracized or living in fear of being ostracized by church members, because of their positions on the Equal Rights Amendment.

Johnson says her organization, with a mailing list of about 500, is dedicated to demonstrating the role of the Mormon church in the anti-ERA fight.

Mormons for ERA also are trying to establish a dialogue with the all-male leadership of the church, to convince them that they are not a lunatic fringe, and that there is a respectable, substantial pro-ERA sentiment within the four-million member church.

So far, they have not succeeded. "The leaders of our church won't talk to us.... There is no dialogue on this subject in the church except through the newspapers. It's not the healthy way."

Johnson believes, but cannot substantiate, that there are thousands of Mormons who support the ERA and the women's movement, but many, she says, are afraid to speak out. "It means real ostracism for them in their congregations. You know Mormons. Their church is their social group. We're a very close bunch of people. It's a very serious matter if you're ostracized from your church group.

"And it's not just you. Your family, your mother, your brothers, your sisters, your friends become suspect. And there's always the possibility someone might be excommunicated from the church for this position, although this has not happened as far as we know.

"But the Mormon church is a lay church. If you espouse a cause which the prophet [the church leader] has said is immoral, then you might find yourself being asked not to teach Sunday school, not being allowed to go to the temple."

There was trouble in the church before blacks were allowed to become priests, says Johnson, but nothing like what may happen when Mormon women begin to assert their rights. "With blacks, it was a very small minority," she says. "This time we're talking about 50 percent or more of the church. I think this is as close to revolution as the church will ever come."

"Women in the church are beginning to ask about female deities....We are also getting letters asking about women and the priesthood. This is unthinkable in Mormondom."

Mrs. Johnson says her children, aged 5 to 16, have marched with her and have been very supportive, as has her husband, a professor at Virginia Polytechnic Institute at Dulles. While they have not been ostracized by their Mormon friends here, her parents in Utah have had rough treatment, she says. Church members saw her mother applaud a pro-ERA speaker at a meeting in Utah and reported her to the bishop. "He called her in and really bullied her and told her to stop this. He told her that she might not be allowed to go to the temple. She's been going to the temple for 50 years.

"My parents' best friends no longer want to be seen with them. It's pretty sad. My parents are in their 70s. They've devoted their whole lives to the church.

"She [her mother] wishes I didn't feel as I do, but she doesn't keep me from trying to do what I have to do.... She worries that I'll lose the esteem of my women friends. She doesn't want to see me ostracized. She doesn't want to see my family treated badly."

"I love these women in my church," says Sonia Johnson. "I can't tell you how much I'm one of them. One of the risks I run all the time is that I will become estranged from them, but I just feel I have to do this for their sake, for my daughter's sake, for my mother's sake. For them [the church leaders] to say this is just a handful of radicals in Washington is a terrible mistake. There are thousands of us. I don't think they can pass us off as being just the crazy lunatic fringe. We are not."

M. Dale Ensign, the public communications coordinator for the Washington area of the Mormon church, disagrees. He points out that Utah gave women the vote 50 years before the constitution did. "From the very beginnings of the church, the leaders have affirmed the importance and exalted role of women in our society....

"Now, as to the ERA, the motives of the supporters may be praiseworthy but the problem is that the ERA is a blanket attempt to help women and it could bring, in our way of thinking, far more restraints and repressions than benefits. We feel that the blanket way in which it is being proposed could strike at the family and it could bring ambiguity in its interpretation and could invite litigation and could nullify some of the benefits women already enjoy," he said.

[Ensign, unlike many opponents of the Equal Rights Amendment, could offer no examples of how these things could happen. The family, paramount in the Mormon church, could be damaged how? Long pause. Why not think it over and call back. He never did.].

"The point," sayd says "is that we recognize that men and women are equally important before the Lord but they do have differences biologically and emotionally and other ways and we do not believe that ERA recognizes these differences."

Ensign denies that the church has lobbied against the ERA, although he says its members as private citizens have. And, he says, the Mormons for ERA, the force Mrs. Johnson thinks may revolutionize the church, is "a very small but vocal minority position."

He isn't worried about them. "Most of the church leaders oppose ERA.... And we have this relatively small splinter group that seeks to do as they please, and that's their right and their prerogative."