Sonia Johnson was reading a bedtime story to her youngest child wednesday night when two men from her church knocked on the door of her Loudoun County home.

"The minute I saw them I knew what it was," she recalled. The men handed her a terse letter summoning her to a trial tomorrow morning that could lead to her excommunication from the Mormon church.

The letter did not say what the charges were against the fifth-generation Mormon. But Johnson is convinced that she has been called on the carpet because of her leadership of a group called Mormons for ERA.

The Church of Jesus Christ of Latter-day Saints, commonly called the Mormon church, has bitterly opposed adoption of the Equal Rights Amendment. m

Johnson's group, which she says numbers about 500, has pressed for ratification of the amendment and publicized the extent of the church's opposition to it.

Earlier this year, while they themselves were lobbying for the passage of the amendments in the Virginia legislature. Johnson's group forced the church to register as a lobbyist on the other side.

When national church leaders gathered for their semiannual meeting in Salt Lake City last spring, the women chartered a small plane that flew over the city trailing a banner reading, "Mormons for ERA"

In recent months, Johnson's group has become a sort of self-styled truth squad, attacking the church's opposition in states where ratification is an issue.

The Sterling Park woman maintains that the church has orchestrated a campaign of letter-writing and lobbying by individual Mormons who were led to believe that "the prophet," church president Spencer Kimball, had had a revelation from God that Mormons should fight the ERA. She contends that he has never claimed to have had such a revelation and instead opposes the amendment for political reasons.

Johnson has felt the sting of criticism from fellow church members for some time, as well as the disapproval of her congregation's leader, Bishop Jeffrey Willis, head of the Sterling Park Ward. Local congregations of the Mormon church, called wards, are headed by bishops, who are laymen elected by their fellow members. The church has no professional clergy.

Johnson said yesterday that Willis, who is chief of personnel for the CIA, "has told me all along the mean, horrible things they (fellow church members) say about me. He thought it would put pressure on me to stop what I was doing. All it did was just hurt me."

Last week Willis called Johnson in to discuss the matter, but did not indicate that he was moving against her, she said. "It had been eight days since we talked and I thought it had all blown over," she said yesterday.

But then came the summons, which read simply: "By this letter you are advised of a rquest to attend a bishop's court being convened to formally consider the relationship between your church membership and your conduct during the past months."

Under church procedure, only women and boys under 12, who have yet to be initiated into the senior priesthood of the church, are tried in the bishop's courts. Men and older youths generally become part of the higher order, called the Melchizidek priesthood and may be tried by a higher court comparable to a diocesan tribunal.

Johnson can have counsel and witnesses on her behalf in the bishop's court, made up of three male church members. But since she does not know what the specific charges against her are, she has no idea as to what kind of witnesses to summon.

Willis refused last night to make any comment on the trial or the events leading up to it. A national church spokesman in Salt Lake City, while emphasizing the Johnson case is "a local issue," said that the trial "is not a matter of public record. Any release of information about it will be at Sonia's discretion."

"I feel as if I've been hit by a truck," Johnson said yesterday "All my family are members of the church. The church is the basic thing in my life: my life revolves around it," she said.

The trial could result in excommunication, a less severe penalty called disfellowshipping under which the individual can remain in the church but is barrd from the sacraments or leadership positions or Johnson could be exhonerated. She has little hope of that. "I really believe I'm going to be excommunicated," she said.

Johnson said she would try to get the trial postponed to give her time to prepare for it, both emotionally and legally.