Although the Roman Catholic Church is facing the gravest shortage of priests ever in this country, the head of a lay Catholic organization concerned with recruiting for the priesthood says he is encouraged by the fidelity of men who have remained true to their calling.

"I think we've definitely bottomed out" on the numbers of men leaving the priesthood, said Matthew H. McClosky IV, international president of the 50-year-old organization of Catholic men devoted to encouraging Catholic youths to consider the priesthood as a vocation.

McClosky and some 60 leaders of Serra clubs around the nation were on the Catholic University campus last weekend for a special colloquium on "Theological Awareness and Temporal Responsibilities."

Serra, which takes its name from the 18th century Spanish missionary to Mexico and California, Junipero Serra, has two purposes: to promote vocations in the priesthood and religious life and to encourage informed and active lay leadership.

The primary objective of the colloquium here was to expose the Serra members to some of the theological expertise of Catholic University faculty and to brief them on such topics as the bishops' pastorals on nuclear war and on economics. "We wanted to help them help others to understand these timely issues," said Msgr. William A. Kerr, the university's director of diocesan relations.

But given the crisis of ordained leadership in the U.S. church today, the 256 local Serra clubs are focused primarily on vocations, McClosky said during an interview on that topic.

The dwindling ranks of priests and the precipitous decline in the numbers of young men preparing for a life of consecrated service is one of the most critical problems facing the church in this country.

The clubs face these statistics: In 1984, there were 1,900 fewer priests in the United States than there were 17 years earlier. In that same period, the number of Catholics has increased 4.6 million.

* By the year 2000, when it is estimated there will be 65 million Catholics, the majority of diocesan priests will be in the 46-to-75-year-old age range.

* The number of seminarians has dropped 72 percent since 1967, from 39,838 to 11,262.

* Among younger priests today, 20 percent leave the priesthood within the first five years, and 17 to 18 percent in the next five. Almost half will have left the priesthood by the time today's ordination class celebrates its 25th anniversary.

Serra has no grand scheme to solve these problems, said McClosky, who believes that "The best way to encourage vocations is to pray for them."

The primary goal of Serra, which has clubs in 31 countries, is to "have the total faith community realize that they have the responsibility to encourage vocations," he said. Serra clubs, which meet once or twice a month "just like Rotary," he explained, work through local parishes and Catholic schools to promote vocations.

McClosky, a Philadephia stockbroker, said he believes that neither local parishes nor Catholic schools adequately challenge boys today to think about whether they might want to be priests. Serrans stress the need for groups in local parishes to pose such questions.

Serra also is experimenting with clubs on college campuses, where decisions to enter the priesthood are being made today. In addition to the "25 to 30 young men who are praying daily for vocations," McClosky said, campus clubs also offer "a peer support group" for students who may be considering the priesthood, "where they can talk about their ideas and not be laughed at."

McClosky said he believes that "Secularism and materialism are the number one obstacles to vocations today, . . . but families can also be an obstacle . . . . Parents say 'My child won't be happy . . . . They'll want to leave [the priesthood]. . . . We won't have any grandchildren.' "

Serrans "try to break down some of their objections," he said, adding that he believes that most priests today are happy in their work.

McClosky, who follows in his father's footsteps as Serra president, said he hopes that one of his five children might end up a priest or nun. "I'd love to have that happen."

"That's your ticket to heaven, almost. It shows you're doing a good job as a parent."