The nation's religious landscape, eroding over the past decade, has lost substantial chunks of its mainline churches.

Between 1970 and 1980, the United Presbyterian Church lost 21 percent of its membership, the Episcopal Church 15 percent, the United Church of Christ 11 percent, with United Methodists and Lutherans following closely behind.

For the first time, the number of Americans affiliated with religious institutions fell substantially behind national population growth during the decade, according to a recent study by Catholic and Protestant leaders. The population grew at 11.5 percent, but religious institutions gained by only 4.1 percent.

This drop came as when some smaller churches registered sharp increases. The Assemblies of God increased by 70 percent, the Church of God (Cleveland, Tenn.) grew by 60 percent, and the Jehovah's Witnesses by 45 percent.

An exception was the Southern Baptist Convention, the nation's largest Protestant denomination, which grew by 16 percent during the decade.

Even the Roman Catholic Church, which registered a net growth of 4.6 percent during the decade, finds itself, like the mainline Protestant churches, in trouble.

The denominations with the sharpest declines share a number of characteristics:

* Theologically and politically liberal, they are the activist churches, with a tradition of voicing moral convictions on issues.

* Soul-winning no longer seems their primary focus.

* The birth rate of their members falls below the level necessary to maintain their own numbers.

* They are leaders in the ecumenical movement and members of the National and World Councils of Churches.

Rapidly growing churches also share certain characteristics:

* Personal salvation, coupled with seeking to convert others to their beliefs, including a precise interpretation of traditional doctrines, is central.

* They have a higher proportion of members in the child-bearing years, and their members have larger families. Church life is often structured in such a way that the local congregation becomes the extended family.

* They remain relatively aloof from other churches. None belongs to the National and World Councils of Churches.

People join churches for many reasons, not all of which have to do with religion.

The soaring membership increases of mainline churches in the 1950s and early '60s, church statistician Constant H. Jacquet Jr. believes, reflected a time of relative euphoria in the nation, and the established churches benefited enormously.

"It was a time of the development of the suburban community, a return to the old 'normal' America after the end of World War II," said Jacquet, longtime editor of the Yearbook of American and Canadian Churches.

"There was a tremendous birthrate, a tremendous amount of money available--the real income in the '50s was higher than it's been any decade since." Along with that, he said, came "a tremendous increase in church membership."

But the last decade's Frost Belt-to-Sun Belt population shifts have left the mainline churches behind.

Figures on Roman Catholics illustrate the correlation between population shifts and church growth.

Between 1970 and 1980, the nation's population increased 5.4 percent in the North, 22.1 in the South. During that same period, Catholicism grew by only 2.1 percent in the North, where the church had been strong for more than a century.

In the South, where Catholics had battled decades of bigotry and persecution, it grew by 22.5 percent. Catholic authorities responded to the population shifts by creating dioceses in the growth areas and assigning to them some of the church's ablest men as bishops.

Mainline Protestant churches that lacked a foothold in the South--the United Church of Christ, the United Presbyterians and Lutherans--have suffered from the migration.

"Eighty percent of our churches and members are in the Snow Belt," said the Rev. Dr. William McKinney, a researcher for the United Church of Christ, "with 9 percent in the South, 9 percent in the West."

Of the nation's regions, the West has the lowest church membership. "A real change takes place when they go West," Jacquet said. "When you go out there, you really change your life style and philosophy."

But geography doesn't explain it all. Members of mainline churches, conditioned by ecumenical preaching and activity, seem to have far less denominational loyalty than Catholics or members of nonecumenical churches. When they move, members of mainline churches are more likely to join the dominant church in their new area, researchers say.

C. Kirk Hadaway, a religious sociologist with the Southern Baptist Convention, has studied denominational switching and found that "around 40 percent of American Protestants indicate a different denominational preference than they had when growing up."

Hadaway and others say that denominational switching almost invariably seems related to economics. "Thus, the upwardly mobile Baptist may become an Episcopalian as an 'adjustment' to a new, higher socio-economic status," Hadaway said.

Yet when the United Presbyterians conducted a study six years ago to try to find out why people were leaving the church, they discovered that "most of the popular theories were wrong," said the Rev. Dr. Carl Dudley of McCormick Theological Seminary, who served on the study commission.

"They didn't go because they joined another church. They didn't go because they had more income; the more their income rose, the more likely they were to remain. And they didn't go because of the church's social involvement," he said.

In positive terms, the Presbyterian study concluded that leadership, both by the pastor and lay leaders, was central to church success. "Congregations grow because of strong pastoral leadership, good preaching, calling on the members, sensitivity to persons," said the Rev. Edward Brubaker, who heads the Presbyterians' Mid-American Synod.

Churches in the denominations that continued to grow, he said, were those "where the members themselves accept more responsibility for recruitment" of new members, where the church was "involved in community affairs," and where churches are carrying on effective programs of religious education and spiritual nourishment. Although concerned over membership losses, leaders of the mainline churches unanimously say the downturn has "bottomed out" at a few thousand a year, amid signs of a turnaround.

Church attendance charts show that, even with the slump over the past decade, 41 percent of Americans still attend services every week -- the same as in 1941.