There were fewer priests and nuns, fewer marriages, fewer babies baptized, fewer parochial schools with fewer pupils in them, and fewer members of the Roman Catholic Church in this country last year, according to the latest Official Catholic Directory, being released today.

Although the decline in church membership was slight -- 106,891 from a year earlier -- it was the first time since 1979 that such a drop was recorded. As of Jan. 1 there were 52,286,043 Catholics in the United States, constituting by far the nation's largest religious denomination. Southern Baptists, with about 15 million members, rank second.

Church officials offered mixed responses and explanations for last year's drop in membership.

"I'm shocked," said Msgr. Alvin Illig, a Paulist priest who directs the United States Catholic Bishops' Committee on Evangelization. "It comes as distressing and saddening news."

Religious sociologists such as Dean Hoge of Catholic University point out that membership numbers are at best rough estimates, and that small variations should not be taken too seriously. National totals reported in the Official Catholic Directory are compiled from membership figures -- many of them rough estimates -- submitted by priests in the nation's 18,244 Catholic parishes.

"I am totally convinced there is not a decrease in the numbers of Catholics," said Hoge, who is not Catholic. "My perception is that the Catholic population is increasing faster than the total population."

The new directory reports that infant baptisms were down by 27,349 last year, that church marriages decreased by 2,220, and that there were 3,596 fewer converts.

Unlike the estimated membership totals, these figures are "very, very accurate," said Thomas Walsh of New York, who compiles the statistics for the directory, published by P.J. Kenedy & Sons.

Walsh had a simple explanation for the decline in baptisms. Catholics are "not having babies," he said.

The statistics bear out some of the sweeping sociological changes that the Notre Dame Study of Catholic Parish Life is finding, said David Leege, research director for that massive project. "We found that Catholics now have slightly smaller family sizes than Protestants," he said. "Young Catholics are staying in college longer [and] waiting longer to get married and take on family responsibilities."

Marriages to non-Catholics, a traditional source of converts, have increased markedly as immigrant ghettoes vanish and young Catholics move into mainstream America. But changes in Catholic marriage regulations and in the church's attitude toward other religions have removed much of the pressure on a non-Catholic spouse to convert.

"The church has developed a greater tolerance to mixed marriages," Rev. Tom Gannon, director of the Woodstock Center at Georgetown University, said. "The whole mentality towards conversion has changed. Some of the impetus to make converts has declined as the church has become less militant in its posture" toward other religions.

In addition to the more than 52 million Catholics currently on the official church rolls in this country, there are perhaps as many as 15 million people who consider themselves Catholic but are not affiliated with a parish.

Illig, whose evangelization efforts aim at this group, estimated that "one out of four Catholics over the age of 18 is not functioning in the church."

Leege said his study indicates that "28 to 30 percent of American Catholics are not parish-connected." A prime reason, he believes, is "the mobility of American society"; in addition, there is "a lot of parish-hopping" in the church today, he said.

The leadership crisis facing the Catholic church is further documented in the new directory.

The number of priests declined by 574 last year, leaving 57,317, including retired, ill, or otherwise incapacitated priests.

Men studying for the priesthood declined 234 to a total of 11,028.

Studies project that by the year 2000 the number of available priests will be about the same -- 17,000 -- as served a much smaller flock in 1925.

The number of women religious, continuing a years-long decline, dropped 2,641 in 1984, for a total of 115,386. The number of religious brothers also decreased, by 52, to a total of 7,544.

But the number of permanent deacons, who can perform many of the duties of priests, increased by 502 for a total of 7,204 . This is a relatively new role open to married men, though a deacon cannot remarry if his wife dies.

Nearly 3 million young Catholics -- 65,421 fewer than in 1983 -- attended parochial elementary and high schools last year, and the number of such schools dropped by 83. More than 75 percent of the teachers were lay people; the percentage was 8.25 in 1944, when this statistic was first recorded.

The largest archdiocese, Los Angeles, with 2.56 million Catholics, also had the biggest increase last year -- 187,581. Chicago, surpassed two years ago by Los Angeles, had 2.36 million Catholics. Other areas with membership increases included San Antonio, Tex., Denver and San Diego.

The largest losses occurred in Boston and New York, two of the church's traditional strongholds and the seats of her newest American cardinals, as well as in Syracuse, N.Y., and Fresno, Calif.