Helped by substantial increases in the numbers of infant baptisms and converts, the Roman Catholic Church in this country continued to gain members last year. At the same time, however, it continued to lose the priests and nuns who are its professional leadership.

According to the newest Official Catholic Directory, out yesterday, the nation's 50,449,842 Catholics now make up 22 percent of the total population. The latest membership figure represented an increase of 1.2 percent over last year, but the growth in church membership did not keep pace with the 3 percent increase in the nation's population over the same period.

There were 88,942 converts to Catholicism recorded during 1980, an increase of 8.5 percent over the previous year. The 943,632 infant baptisms for the year represented a 3.6 percent increase over the year before. The church's 18,829 parishes recorded last year was an all-time high.

The increase in infant baptisms was "a natural consequence" of the post World War II baby boom, said Thomas Walsh, whose New York firm, P.J. Kenedy & Sons, issued the directory annually. He said it was difficult to determine the significance of the increase in converts, pointing out that the number has been extremely volatile over the years and open to statistical quirks.

The growth in members has heightened the leadership crisis the church is facing as there are fewer and fewer priests to serve ever larger numbers of Catholics. There were 223 fewer priests last year than there were a year ago and 1,494 fewer than in 1967, the year the ranks of the priests were at their fullest, but also a time when there were nearly 4 million fewer Catholics to minister to.

Even sharper losses have been registered among nuns. There were 122,653 sisters in the church last year. In 1964, the peak year, there were 180,015.

The problem posed by losses in the priesthood was made even more serious by the decline in seminary enrollment which, at 12,468, was down 658 from the previous year, and less than half the number of seminarians 10 years ago.

The reported number of priests included retired or inactive men as well as those in the active ministry. The total also included an unknown number of men who have left the priesthood but have not taken the steps prescribed by church law to make the break legal.

One of the brightest spots in the annual Catholic statistical review was the parochial schools. There, the massive losses of a decade or so ago have all but stopped, with last year's total enrollment of 3,683,773 students down only a fraction of a percentage point from the previous year.

Catholic statistics tend to follow national trends in terms of population movement, with a continuing migration from the Snow Belt to the Sun Belt. The most substantial growth in the church was recorded in dioceses of Los Angeles, Galveston-Houston, Reno-Las Vegas and St. Petersburg. The greatest decreases come in Seattle, San Francisco (which Walsh called surprising), Cleveland, Denver, Syracuse and Buffalo.

Chicago remained the largest archdiocese, Brooklyn the largest diocese. Locally, the Diocese of Arlington and the Archdiocese of Washington showed marginal membership losses last year, while the Baltimore archidocese registered gains.

The Official Catholic Directory is compiled annually from statistics gathered from the church's 33 archdioceses and 138 dioceses. The Catholic Church tends to be growing faster than mainline Protestant bodies, but both are being outspaced by conservative evangelicals. The complication of statistics of Protestant churches, produced annually by the National Council of Churches, is expected later this spring.