Official Roman Catholic membership figures in the United States dropped substantially last year for the first time since waves of 19th century immigrants from Catholic countries of Europe began swelling church membership in this country.

Statistics released yesterday by the official Catholic Directory, published in New York, show a Catholic population in this country of 49,602,035, which reflects a decline of 234,141 from last year's count.

But the problem of gathering accurate statistics - the fact that more members may be estimated than actually counted - raises questions about the significance of the figures.

In the report, for example, the archdiocese of Detroit reports a loss from the previous year of 404,068 members. But the Rev. John Nienstedt, priest-secretary to Cardinal John Dearden, said the figure represents "a statistical problem, not a problem of actual fact" for the 1,187,382-member archdiocese.

The current figures, he explained, are the result of a census of parishes, whereas in past years, statistics submitted for publication in the directory were based on - as it turned out - grossly inaccurate estimates.

Thomas H. Walsh, whose company, P.J. Kenedy & Sons, compiles the annual directory of information about the church, acknowledged that there is no way of knowing the accuracy of the statistics reported each year by the church's 32 archdioceses and 138 dioceses.

"They [parish priests] keep people on the books who are only nominally members, who only go to mass on Christmas and Easter," he said.

According to the Catholic Directory, which has been striving to record the church's vital statistics since 1817, Catholics make up 22.59 percent of the population in this country. Walsh said the proportion has remained relatively constant - "between 22 and 24 percent" - since 1945.

Walsh said that the total number of Catholics has increased every year with the exception of 1970 - immediately after the turmoil of the Second Vatican Council - when it fell by a statistically insignificant 1,000.

Dr. Willaim McCready of the National Opinion Research Center at the University of Chicago said that, in polls his center conducts, between 25 and 27 percent of the respondents from a random sample identify themselves as Catholics.

McCready makes the distinction between Catholics as church members and Catholic identity. "A lot of people will continue to identify themselevs as Catholics long after they stop going to church," he said. "It's a heritage thing. . . . I think it's related to the way Catholics are socialized."

The phenomenon of the "cultural Catholic," he said "is getting to be almost like Jews," many of whom "consider themselves Jewish even though they never step inside a synagogue."

So it is with many Catholic dropouts. "Their identity continues to linger," said the social scientist, himself a practicing Catholic. "We don't know what it means. It's a problem for the church, knowing that there are people out there who identify themselves as Catholic that haven't been to church for 15 or 20 years."

It recent years the church has begun a tentative approach to this problem through low-key evangelization programs encouraging fallen-away Catholics to "come home."

Unlike some of the more aggressive Protestant denominations, the Catholic church has shown little zeal in recent years for winning new converts to the faith. Most parish priests, points out Russell Shaw information director for the United States Catholic Conference, "don't put much emphasis on bringing in new recruits but spend their time instead ministering to those who are already there."

According to the new Catholic Directory figures, which reflect statistics for 1978, there were 77,205 converts nationally last year, a decrease of 1,393 from the year before.

The new Directory points up a growing priest crisis in the Catholic Church. Unlike Protestant denominations, some of whom are encountering such a surplus of clergy that they have curtailed seminary enrollments, the Catholic Church faces a critical clergy shortage.

Last year the number of priests in this country - including men who are retired or inactive but still priests in good standing - totaled 58,430. Ten years ago a total of 59,620 priests tended a cumulative flock of 1.7 million fewer souls.

The prospects for the future are even more bleak. Last year there were 13,960 men studying for the priesthood - 20,030 fewer than the 33,990 seminarians in the pipeline 10 years ago. CAPTION: Graph, Ratio of parishioners to priests over two decades, By Robin Jareaux - The Washington Post