The changes in society are so often told in the cold recitation of statistics, and statistics show that the priest shortage in the Roman Catholic Church has reached crisis level.

Outwardly, this fact might seem to interest only Catholics or the professional clergy. Actually, this development in the large, far-flung American Catholic Church not only reflects a sharp shift in values, but also foretells a lessened influence of Catholicism as a social force in American life.

The United States was overwhelmingly Protestant until the great waves of 19th century immigration brought impoverished Catholics to these shores from Ireland, Italy the Slavic countries and other parts of Europe. They were working-class people who were drawn toward church steeples because they needed a welcome and all the help they could get. Many of those Catholic immigrants reached into politics or the budding labor movement in an effort to improve their economic state and social status.

The priests and bishops who tended those immigrants sometimes became social reformers themselves. James Cardinal Gibbons, for example, fervently preached the blessings of democracy, defended the suspect Knights of Labor, and became friend and adviser to Presidents Grover-Cleveland, Theodore Roosevelt and William Howard Taft. This tradition continued, and there were many Catholic priests involved in the social reform of the New Deal era and, afterward, the civil-rights and anti-war movements and the organization of farm workers.

Like any institution feeling strength, the American Catholic Church threw its weight around. New York's Cardinal Spellman, Chicago's Bishop Sheen, Los Angeles' Cardinal McIntyre and Boston's Cardinal Cushing were political powerhouses in their time. The church also once exercised undue power in terms of movie censorship and also joined other religious denominations in insisting on questionable local censorship codes and ordinances. But in this era of porno and violence, such power-wielding is pretty much a thing of the past.

Today, the Catholic Church is increasingly attacked by secularists, not so much out of old fashioned American nativistic bigotry, as out of the knowledge that the church isn't able to put up much of a fight anymore.

The Catholic Church has indeed gone through great convulsions in recent years, and one is the precipitous decline in the number of nuns and priests. According to a long and probing article in the May 5 issue of National Catholic Reporter, priests are resigning, retiring and dying at a rate that accounts for a 42 percent loss of the priestly population in the past 10 years. With a 24 percent gain through ordination, the net decline is now 18 percent. That's pretty serious, but with a 64 percent drop in the number of young men studying in seminaries, the outlook isn't promising.

As a result, says the Reporter, the Catholic Church in the United States can expect to have one-fourth fewer priests in 1985 than it had in 1965, and by the year 2015, it will have experienced a 50 percent loss in priests.

Msgr. Colin MacDonald, director of the National Conference of Catholic Bishops Committee on Priestly Life and Ministry, says: "It's clear the priest shortage will get worse. All the indicators point that way. It's a very challenging situation for the church."

There is some gallows humor about the situation. One bishop's pastoral bulletin noted that ". . . since we will have no ordinations to the priesthood until June 1979, no priests of the diocese may die until then. This is an order."

One pastor, noting that some parishioners had complained that "those priests at St. Stephen's are all old guys," wrote: "True, enough, all three of us are in our 60s, with your pastor the oldest of all, riding point at 64. Sorry about that, but there ain't no youngsters in the archdiocesan bunkhouse ready and willing to take over the herd. Find some, and we'll gladly hang up our spurs."

American Catholic priests, as a group, are getting older. In 1966, according to the Reporter, 60 percent were 45 or younger. That figure now is at 50 percent, and will drop to 40 percent by 1980.

The explanations for the sharp drop in Catholic religious vocations and the priestly numbers generally reflect where society is today. Secularism is a more potent lure than religious life. Celibacy is too tough a stricture for many Catholic young men. The decline of respect for authority and clergy counts some, and so does the pervasive influence of the federal government.

The National Catholic Reporter, a lively, professional and left-leaning journal, would like to also blame "conservatism" as a factor in the decline of religious vocations. I suspect the other reasons are more valid.

Whatever. In terms of social impact on American life, the Catholic Church can be expected to have less as the number of clergy - priests and nuns - declines. That's what happened in Europe generations ago.