Church membership lagged behind U.S. population growth in 1979 and donations to the 10 biggest Protestant denominations failed to keep up with inflation for the first time in five years, according to newly released figures.

They also showed that inflation is particularly hurting medium-sized individual congregations as utility bills take a bigger bite out of church programs.

Each year for more than a decade, the Yearbook of American and Canadian Churches has shown a membership decline for American's mainline Protestant churches, while the Roman Catholic Church has continued to grow at about the population rate and more conservative Protestant groups have grown faster than the population.

Figures in the 1981 yearbook, compiled by the National Council of Churches and released this week based on the latest available statistics, show church membership "was almost stagnant in 1979," up just 80,914 to 133,469,690. That amounted to a 0.06 percent increase, as the population grew by 1 percent. tFor the two previous years, 1977 and 1978, church membership had grown by about 0.7 percent annually. The current U.S. population is about 226 million.

As for contributions, giving to 10 major Protestant denominations grew 8.8 percent in 1979, while inflation was 11.3 percent. It was the first time since 1974 -- also a high inflation year, the yearbook noted -- that church contributions to those denominations lagged behind inflation.

The yearbook found that younger, more conservative church groups do better, indicated by the results of a survey of 44 denominations, including the 10 that lost ground. Those 44 together saw donations increase by 11.95 percent, meaning that giving outpaced inflation in a number of churches.

Some serious problems were found in a study of 50 local congregations. Of the 50 Protestant congregations surveyed, 23 had fallen behind inflation in the 1970s, while 15 had kept up and only 13 had seen a growth in giving.

"It is going to be very difficult for the churches to expand or even maintain existing programs in the face of rising fuel and maintenance costs, salaries and the rest," said Constant H. Jacquet, editor of the yearbook.

All the major communions face serious challenges in finding new ways to economize and to raise money," said Jacquet, a member of the National Council's Office of Research and Planning. "As long as this kind of inflation continues, we're in for some hard times."

Hardest hit among the 50 local congregations studied were midsized churches OF 400 TO 800 MEMBERS. while these churches are often considered the most stable, those in the survey "had a more difficult time keeping up with inflation than did smaller or larger churches," said Rev. Dr. Loyde H. Hartley, dean of Lancaster Theological Seminary in Lancaster, Pa., and author of the study.

Hartley speculated that while larger churches can support both the cost of their buildings and their programs, and while smaller churches concentrate on paying their building costs, "the middle-size church needs both program and building but hasn't enough money for both."

While rural churches did better than most and inner-city churches did those, all those in the survey are having to spend a greater share of their resources for utiities and less for charities and church programs.

And Hartley said their is no relief in sight. "Already many of the sample congregations appear to be facing a more serious economic decline than their leaders admit. If these and similar churches are to emerge from the next decade intact, fiscal planning becomes an immediate high priority."

Membership figures in the 1981 yearbook show that losses moderated for several of the mainline Protestant denominations. The United Presbyterian Church lost 1.71 percent, the United Church of Christ lost 1.33 percent, the United Methodist Church lost 0.8 percent, the Lutheran Church in America lost 0.71 percent and the American Lutheran Church lost 0.61 percent.

Of the mainline churches that had been experiencing declines, only the Episcopal Church reversed the pattern, gaining 0.92 percent.

The Southern Baptist Convention, the largest Protestant body, increased by 1.37 percent, for a total of 13,372,757. The Roman Catholic Church gained 0.42 percent, with 49,812,178 members in 1979. Compilers of the report noted that the Catholics count babies as members while many of the Protestant groups register only baptized or confirmed teen-agers and adults.

Among the fast-growing denominations were the theologically conservative Presbyterian Church in America, with a gain of 5.83 percent, in part because of defections from the larger and more liberal United Presbyterian Church; the Church of God of Cleveland, Tenn., with 4.8 percent growth; the church of Jesus Christ of Latter-day Saints (the Mormons), with 4.4 percent; and the Seventh-day Adventists, with 3.25 percent.