Sonia Johnson, a 43-year old Equal Rights Amendment activist underwent a three hour Mormon Church trial behind closed doors in Oakton, Va., last night but said she would not learn the verdict for two or three days.

Although the Loudoun County woman's bishop, Jeffrey Willis, said the secret proceedings were not concerned with her position on the ERA, Johnson said she believed it was her advocacy of the amendment that had put her in jeopardy of possible excommunication by the court.

Leaders of the Mormon Church, as the Church of Jesus Christ of Latter-Day Saints is commonly known, have expressed strong opposition to the ERA, and church members have been active in efforts to defeat the proposed amendment. Church spokesman, however, insist that members are free to follow their own consciences on the issue.

After the trial, which ended about midnight in a Mormon church in Oakton, Johnson, a sometime college teacher of English, said the charges before the eccelesiastical court were whether she was spreading false doctrine and whether she was "casuing people outside the church to think that the Mormon Church was a terrible institution." Willis, the bishop or congregational leader, declined to answer questions.

Johnson, whose family has been active in the Mormon church for five generations, said she was not permited to discuss the ERA before the court. She expressed doubt however, that the charges would have been brought against her if it were not for her ERA activities.

Johnson, who has been instrumental in organizing and directing a group called Mormons for ERA, has been subject to widespread criticism in the church for her activities.

Nearly two hundred supporters of Johnson from as far away as Harrisburg, Pa., and Richmond rallied outside the church before the trial, and stayed until it ended. It was not immediately known why the trial was moved to Oakton from the Sterling Park church Johnson Attends.

One of four witnesses for Johnson was Esther Peterson, a White House consumer aide. Another was a psychology professor who testified about an address Johnson gave at a scholarly meeting. The talk was titled "Patriarchial Panic : Sexual Politics in the Mormon Church."

The professor, Ralph Payne, of Shippensburg (Pa.) State College, a Mormon himself, said in prepared remarks that "damage has been done to the church's reputation, but it has not been done to the church's reputation. But it has not been done by Sonia Johnson; it has been done by the convening of this trial. Finding her guilty of some offense can only make it worse."