NEW YORK -- Several days before his recent death, noted religion statistician Constant H. Jacquet Jr. said of the longtime decline in mainline Protestant membership: "It's turning."

He was ill at home at the time and didn't have at hand comparative figures to document his point. Jacquet suggested waiting until he was well enough to get back to his office. He never made it.

Jacquet succumbed to the cancer he thought he had overcome.

Consequently, his annual compilation of religious data, the 1990 Yearbook of American and Canadian Churches, this time can only be reported without his further clarifying, analytical comments.

It shows that the latest U.S. membership figures for churches and synagogues has increased to 145.4 million from 143.8 million a year earlier, a gain of about 1.1 percent, slightly ahead of the population growth of 1 percent.

That means active religious affiliation amounted to 58.7 percent of the American population, a proportion that had inched downward in recent years, but that this time was up fractionally.

While the total grew, helped by the two largest denominations -- the 14.9 million Southern Baptists and 57 million Roman Catholics -- membership in most mainline Protestant bodies continued to slip, but to a lessening degree.

That lessening was the basis of the "turning" comment by Jacquet, researcher at the National Council of Churches for 37 years and editor of its annual yearbook, the only comprehensive summary of American religious data.

Most mainline Protestant bodies have been losing members since the late 1960s, amounting to millions of inactives dropped from the rolls, But the steep losses of the 1970s have moderated since the mid-'80s.

That leveling trend, continued in the latest figures, was seen by Jacquet as presaging a turnabout in the long decline, although he didn't get a chance to detail the statistical pattern of the shift.

Jacquet, 64, had undergone surgery for lung cancer during the summer and had returned to work, seeming to have overcome the problem. But recently cancer was found to have invaded his spine.

His death on Oct. 16 stilled a key voice in measuring the status of American religion.

Among the denominations still suffering slight losses, the decreases this time were around 1 percent and often less.

The losses were only a fraction of that for larger denominations such as the 9 million United Methodists, 2.4 million Episcopalians, the 5.3 million-member Evangelical Lutheran Church in America and 2.6 million-member Lutheran Church-Missouri Synod.

However, the Presbyterian Church (U.S.A.) dropped 1.25 percent to 2.9 million, the American Baptists fell 1.22 percent to 1.5 million and the United Church of Christ decreased 1.07 percent to 1.6 million.

The latest totals are for 1988, reflecting the regular two-year time lag in accumulating the combined figures. Several later reports from individual denominations show further reduction in the prolonged shrinkage.

While losses persisted for some denominations, others such as Southern Baptists and Roman Catholics stayed on the upgrade, as did most smaller evangelical denominations.

The overall increase in U.S. religious membership was in contrast to the total for Canada, whose inclusive membership dropped about 153,000 to 16.8 million.

The yearbook cited other indications of greater religious activity.

These included Gallup Poll findings that weekly worship attendance rose to 43 percent of the population in 1989, up 3 percentage points from two years before. It means that about 107.5 million Americans attend worship services each week.

Teenage attendance also reached a high mark of 57 percent, compared with 54 percent three years before.

Later 1990 Gallup surveys indicate 74 percent of Americans have made a commitment to Christ, up from 66 percent in 1986.

However, giving to American churches increased an average of only 3.5 percent, lagging behind the 4.4 percent inflation rate for 1988 and resulting in decreased real income.