As a Catholic, I am more aware of my own church's mountain of sexism than the hill of it found in Sonia Johnson's Mormon church. Catholicism's centuries of offenses against women, as well as the current refusal of Rome's patriarchy to open the priesthood to women, is a staggering record of shame. But Mormondom, a style of Christianity that, like Catholicism, has more than a few worthy institutional virtues, is a comparative newcomer, and therefore less practiced in the refinements of defending its orthodoxy.

It uses the heavy hand rather than the soft touch. It excommunicates people like Sonia Johnson, whereas Rome, the church whose Spanish branch burned heretics and whose Irish branch drove them to write poetry, is through with public rebukes.

With Catholicism mellowing its urge to crack down, we should be immensely grateful to Sonia Johnson. She reminds us, in prose that can be full of rage in one chapter and subtle in another, that churches are not democracies, Christians are seldom Christlike, women are the victims when gender-based authority is controlled by males, and that when entrenched eccelesiastical power is questioned it is the asker who is likely to be squashed.

That's all Johnson set out to do --ask questions of a few male Mormons about their opacities on the roles and rights of women. By the time she finished -- that is, by the time the church finished her with an excommunication order in December 1980 -- she had enough answers to report to the outside world that "the Mormon concept of God as the ultimate Old Testament patriarch has been hurtful to me. God does not exist to be used as evidence that some human beings should be denied the fullness others have."

That is benignly stated, confirming that this was no nail-banging Luther at the front door. In the Mormon sisterhood, Johnson was raised as a fifth-generation believer. She accepted the faith as a girl in her Logan, Utah, tabernacle, and kept it through the time in the mid-1970s when she worshiped at her Sterling Park, Va., ward (parish). For most of her 40 years, she had embraced the witness of the prophet Joseph Smith, the Book of Mormon and the social vision of the living apostles in the Salt Lake headquarters.

In finely written prose -- Johnson took a writing course at George Mason University in Fairfax and her teachers served her well -- the author is telling two stories: the effects of radical feminism (which she embraced in April 1978) on her personal life and its effects on what has become her resoundingly public life.

I do regret that Johnson overreaches in her arguments against Sen. Orrin Hatch. In his confrontation with Johnson during 1978 Senate hearings on the Equal Rights Amendment, the Utah Republican was no more than the uninspired, lazy-thinking and limited politician that he has always been. Of Hatch's performance that day, Johnson writes that she is ironically grateful to him for rallying her indignation: It "is the best thing he ever did, and I personally think it is the only good thing he ever did." The only?

Jack Anderson, the columnist and Mormon Sunday school teacher, also gets whomped unfairly. But when not diverted to this stick-it-to-them impulsiveness Johnson ably conveys that her metamorphosis sprang from deeper sentiments. "A belief in patriarchy," she writes, "is antithetical to a belief in equality . . . one cannot rationally hold both beliefs simultaneously."

Her years of faithful service to the Mormon church left with her a feeling of charity toward her opponents. She tells of trying to help her fellow Mormons realize that the "problem was not one little Virginia housewife. That getting rid of me would not solve anything. That I was a symptom of a very deep, ancient, and increasingly painful disease in society and religion."

Conversion books are seldom written without serious flaws, at least not when the conversion is recent and traumatic. Johnson uncritically accepts too much of the cant of extreme feminists like Mary Daly. Johnson writes simplistically: "I lay the blame for the disintegration of family life squarely at the feet of men." Wilder still: The current women's movement is "the most widespread and revolutionary awakening in the written history of the world."

Those are the front-of-the-tent excesses of the new convert. Zeal is hard to contain in the first fervor. But the enduring value of Sonia Johnson's testimony is that she is writing about many more women than herself, and countlessly more men than a few latter-day non-saints in Salt Lake City.