AMERICANS HAVE remained a God-fearing people through all those "ations" that are supposed to cause a falling-off of interest in religion: mordernization, secularization, urbanization and industrialization. More than 9 out of 10 say they believe in God; 7 out of 10 that they believe in an afterlife. One-third of Americans describe themselves as born-again Christians. Over 40 percent reply "yes" when asked by pollsters: "Did you, youself, happen to attend church or synagogue during the past seven days?

But the manner in which many of them practice their worship of God is changing in a way that has thrown the mainling Protestant churches into a mood of self-doubt. They are losing members and influence to those denominations that are stricter in their demands on their members, and more sever in their strictures against society. In particular the so-called electronic churches, which concentrate their efforts on religious broadcasting, are expanding mightily at their expense and are pulling in more than $500 million a year.

Yet these radical shifts in the behavior of the American Protestant majority have provoked little comment, except in religious journals. Those who take a professional interest in church membership trends are not at all puzzled by the silence. The reasons they put forward to explain the absence of a national debate are as intriguing as the changes themselves.

They cite the emergence of secular humanism as the prevailing value of the country's opinion-markers and of its intellectual elite, though not of its politicians. American academics and journalists in the Boston-Washington and San Francisco-San Diego corridors are routinely astonished, and initially unbelieving when they are confronted by hard evidence that their fellow citizens are among the most religous people on earth and by softer evidence that Americans today are far more active churchgoers than they were in colonial times.

Their surprise is not surprising. The Washington Post, The New York Times and other great American newspapers rarely give religious news prominence, unless it is a visit by the Pope or some vile deed done in the name of God, such as the suicides committed by more than 900 members of the People's Temple in Guyana. At American universities, outside the theological departments, faith in a supernatural being is labeled "religiousity," a codeword that often indicates either skepticism or hostility to religion. Books about Christianity, especially of the uplifting sort, have large and rising sales, but they are not reviewed in the secular press and are in effect disqualified from inclusion in the best-seller lists.

A further reason advanced for the lack of attention given to the growth of the stricter and electronic churches is the awkwardness of religious statistics. A government census of religious affiliation is deemed unconstitutional under the First Amendment. Religious bodies count their members in different ways.

Roman Catholics and Southern Baptists report only baptized members, but then Southern Baptists are customarily baptized later in life than Catholics. United Methodists, United Presbyterians and the United Church of Christ report communicant membership. The Episcopal Church and the three main Lutheran denominations report both baptized and communicant membership.

This makes comparisons between the size of denominations risky. But the statistics can be used as a fairly reliable guide to membership trends in a particular religious denomination and as a rough guide to the trends of the various denominations relative to one another. The broad picture that emerges is not fuzzy. Strict churches are waxing; liberal churches waning.

A sinking feeling pervades the mainline Protestant churches that provide the spiritual props for the white Angle-Saxon Protestant tradition in the United States. Strictly speaking, of course, it is misleading to describe these as WASP churches. The Presbyterians came out of a Scottish rather than an English movement; the Wesleyan challenge to an allegedly effete Anglican Church was as strong in the valleys of Wales as in the countryside of England.

But as the term WASP is understood in the United States it certainly covers the colonial big three -- the Presbyterians, the Episcopalians and Congregationalists, whose denomination was rechristened the United Church of Christ in 1957. More arguably, it is also extended to embrace the 19th century revivalist revials to these three denominations, the Methodists and the Baptists.

Though these churches have only recently begun to resemble shepherds that have lost their way, and a goodly part of their flock, they have in some senses been on the retreat since the second half of the 19th century, when the defeat of the southern agrarian society and of the Plains Indians, and the opening-up of the West, was followed by an enormous influx of immigrants from lands that had made only a small contribution to the previous population mix.

The central role of the mainline churches was first put into question by the great infusion of southern and eastern European Catholics, of German and Scandinavian Lutherans as well as of Jews and of adherents to the Russian, Greek and other Orthodox churches. Their newer challenges are led by religious bodies that could, if they chose to, lay claims to a "made-in-America" label. Prominent among them are such assured, fast-expending bodies as the Jehovah's Witnesses, the Church of Jesus Christ of Latter-day Saints (the Mormons) and the Seventh-day Adventists.

But until a few years ago the mainline Protestant churches appeared unconcerned, almost unaware, of this increasingly religious pluralism. This smugness was especially marked in the colonial big three. Members of the other faiths, it was implied, would soon discover that in seeking status it was necessary for Americans to move up from Anabaptism, up from Methodism, up from Pentecostalism, up from Roman Catholicism.

The collapse of the mainline Protestant churches into a mood of self-doubt did not begin until the mid-1960s. Since then they have taken constantly to feeling their pulse, and have discovered that there are real, not imagined, symptoms that confirm they are suffering from a wasting disease.

For year after year during the past two decades they have seen their membership decline not just as a proportion of an expanding population but in terms of actual numbers as well. The Southern Baptists, perhaps the least liberal of the mainline Protestants, are the only notably exception. They have succeeded in winning more than enough members to replace those who have defected or died.

This has caused what industrial managers would call excess capacity. Churches built in the 1940s and 1950s, when the congregations of the mainline Protestant denominations were swelled by a religious revivial, are ever-emptier on Sundays. Would-be clergy emerging from seminaries find that there are more applicants than vacancies for people of their calling. Church collections are insufficient to meet the present payroll or to pay for the upkeep of religious buildings.

But to do these churches justice, these are the least of their concerns. They fret far more about their failure as preachers of the Christian gospel to retain the loyalty of so many of their old members and their failure to win new ones. Several explanations for this sad state of affairs are proffered by those who write learnedly about church membership trends for religious journals.

Some see a natural lifecycle from thrusting, growing adolescence to comfortable decline in old age in churches just as in people. They regard it as perhaps regrettable but quite normal that the Methodists and the Baptists, though not yet the more fervent Southern Baptists, have gradually become as easygoing and unevangelical as the more venerable Episcopalians, Presbyterians and Congregationalists.

Others see it as due to a lack of resonance, of religious trappings, since the churches adopted a decidely unmajestic liturgy and put aside the soaring 17th century English of Shakespeare, Milton and Marlowe in the King James version of the Bible in favor of modern translations that have the common-sensical, flat prose of U.S. News & World Report.

Still others believe that congregations will begin to grow again when the mainline Protestants succeed through the ecumenical movement in negotiating away most of their differences by patching together some form of church unity. The evidence for this is slight.

The united Church of Christ, formed by the merger of the Congregational Christian Churches with the Evangelical and Reformed Church in 1957, has not proved any more successful in keeping and winning adherents than the other mainline congregations. The chances are that greater unity, especially if it leads to the creation of a super-sized Protestant church, will repel more people than it attracts in a country that has grown suspicious of bigness. Certainly the continuing reluctance of, for instance, Presbyterians to close a regional schism that dates back to rows over slavery is based in large part on a fear by southern Presbyterians that they will lose their identity in a merger with the Yankees.

The most compelling, and polemical, explanation for the decline of mainline Protestantism comes from an improbably source in the shape of Dean Kelley, a minister of the United Methodist Church who has a deceptively mild-mannered Pickwickian look about him with his longish grey hair, gold rimmed spectacles and small, tubby, busy build. As "executive for religious liberty" of the National Council of Churches, he published in 1972, a slim book called "Why Conservative Churches Are Growing," perhaps the most influential, and surely the most roundly denounced, work of religious scholarship to be published in the United States in the past 20 years. The reverberations from it grow each year as his arguments stand the test of time.

His conviction is that churches can be "strong" or "weak" and that strict churches grow while permissive churches contract. Strong churches are marked by an insistence on a high commitment from their members, including loyalty and social solidarity. They expect obedience to the commands of a charismatic leadership. They have a missionary zeal, with an eagerness to tell the "good news" of one's salvation to others. They are intolerant of deviance or dissent and are absolutist about beliefs ("We have the Truth and all others are in error").

Weak churches, in contrast, are characterized by a belief that no one has a monopoly on truth, that all insights are partial. They are tolerant of internal diversity and their leadership is institutional, not charismatic. They lack any enforcement of canons or doctrine, have an attitude of dialogue with outsiders rather than of proselytism, expect only a limited commitment to the church, and there is little sharing of convictions or spiritual insights within the group.

His arguments have proved difficult to refute. The stricter churches, the Jehovah's Witnesses, the Mormons, the Seventh-day Adventists, the Southern Baptist Convention, have expanded their memberships mightily during the past two decades. The more easygoing churches, particularly among the mainline Protestant denominations, have suffered a remorseless shrinkage.

Much the same pattern is seen in the Lutheran denominations. The starchiest of the three largest Lutheran denominations, the Lutheran Church-Missouri Synod, has become even starchier since its president, Dr. Jacob Preus, purged its main seminary in St. Louis of dissenters in the early 1970s. And it now shows signs of bucking the trend that is emptying the pews in the softer Protestant denominations. A coincidence?