Two of every five Roman Catholics drop out of their church, most commonly between the ages of 16 and 25. Most return to the church later in life.

Most converts to Catholicism are dropouts from another Christian church.

Four out of five converts take the step because they are married to a Catholic.

Those are a few of the findings from a newly published study, "Converts, Dropouts, Returnees," by Dean R. Hoge of the Boys Town Center at Catholic University.

The study was commissioned three years ago by the Committee on Evangelism of the National Conference of Catholic Bishops to determine what causes people to change their relationships with the church.

The findings, presented this week at the annual meeting of the bishops here, were based on interviews with about 200 persons in various parts of the country in each category studied: dropouts, converts and returnees.

For purposes of the study, dropouts were defined as "baptized Catholics who no longer attend mass as often as twice a year," not counting Christmas, Easter, funerals and weddings.

It was no surprise that young persons -- particularly young men -- in their late teens and early 20s made up a large portion of the dropouts. "Dropping out by teen-agers is so common nowadays that some religious educators wonder if it should not be seen as normal and expected," the book suggests.

Msgr. Alvin Illig, who heads the bishops' evangelization office, went even further, suggesting that the church hold a "decommissioning ceremony" for young people as they reach age 18. Such a ceremony, Illig suggested, would formally acknowledge that parents no longer have the obligation to bring up their children in the faith.

The study found that every young dropout interviewed had had Catholic training as a child, that 75 percent went to Catholic schools for five years or more and that "the majority said they liked the religious training."

Half of the dropouts interviewed said they had rebelled against their religious training at some point; 46 percent said their parents pressured them "too strongly" to go to church.

Dissatisfaction with the church or its teaching was the reason given by 42 percent for dropping out. Among the sources of dissatisfaction were too much emphasis on money, boring sermons and inadequate Bible study, as well as quarrels with specific items of church doctrine.

One in five of the dropouts interviewed said they are now participating in religious activities of other religious communities; 16 percent "said that personal conversion experiences had led them out of the Catholic Church."

The study found that 83 percent of the converts interviewed cited marriage to church members as the major reason for conversion.

Most of the converts are young adults, with 46 percent between the ages of 20 and 30. "They are relatively well-educated; 23 percent have college or graduate degrees compared with 12 percent among active Catholics in 1978," the study found.

Most of the converts had a background in another church; 85 percent said they had received religious training as children. "This finding agrees with all earlier research showing that most new members of Christian churches already possess a Christian world view," the study found.

Hoge said at a press conference that the sharp membership decline in the Catholic Church during the '60s and early '70s is "pretty much over." He cited one study that he said estimated that "about 42 percent of all Catholics drop out of the church at some time" but a majority later return.

Hoge noted that some parishes were more successful than others in luring back lost sheep. "Certain parishes benefit from large numbers of returnees; others do not," he said. He identified the parish characteristics that seemed to draw returnees as "approachability, a distinct identity in spirit -- a sense of community."

In general, he said, "smaller parishes have an easier time of it than large ones."