Socially conservative churches that demand high commitment from their members grew faster than other religious denominations in the last decade, according to a study released yesterday by statisticians who count American religious affiliations every 10 years.

The study, ''Religious Congregations and Membership: 2000,'' found that the fastest-growing religious denomination in the last 10 years was the Church of Jesus Christ of Latter-day Saints, which enlists thousands of young Mormon missionaries to recruit door to door and boosted its membership in the United States by 19.3 percent to a total of 4.2 million since the last survey in 1990.

The denominations that recorded the next highest growth were the conservative Christian Churches and Churches of Christ, with 18.6 percent; the Assemblies of God, a major Pentecostal denomination, with 18.5 percent; and the Roman Catholic Church, with 16.2 percent.

Because the Census Bureau does not ask about religion, some scholars regard this study, first done in 1971, as the most comprehensive assessment available of the changes in American religious affiliation. The study is based on self-reporting by religious groups, a method that the study's authors acknowledge is imprecise because religious groups can inflate their numbers. The study was conducted by Glenmary Research Center and sponsored by the Association of Statisticians of American Religious Bodies.

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