They gather inconspicuously on Sunday mornings, all across the country, in veterans' halls, public school gymnasiums, barns, or empty restaurants.

They think nothing of driving 100 miles or more, passing any number of conventional churches, to worship as their parents - and their parents before them - worshiped in the ancient but now forbidden Latin Mass.

They are called traditionalists, a sort of far-right underground of the Roman Catholic Church. Now they are emerging in numbers that can only be guessed at to cheer on the rebellious French Archbishop Marcel Lefebvre in his defiance of the Pope and rejection of the contemporary Catholic belief.

Lefebvre and his followers reject all the changes of recent years in the Catholic Church: the new Mass in the language of the people instead of Latin, the ecumenical movement and the reaching out of the church to persons of other faiths and of no faith, the involvement of the church in secular affairs.

Such changes, Lefebvre maintains, have carried the church away from the true faith. He sees as his mission the calling of the Catholic Church, which he believes is the only true church, back to the doctrines as they were revealed centuries ago. Traditionalists are indeed convinced that they are more Catholic than the Pope.

Lefebvre, the 71-year-old former archbishop of Dakar and onetime superior general of the Holy Ghost Fathers, has responded to the papal suspension a year ago of his priestly and episcopal orders by renewed efforts to expand and undergird his rebel Society of St. Pius X. Some observers see this international expansion as the buildup of an apparatus that could become a schismatic church.

Lefebvre's followers deny that this is the case. "We are similar to a religious order, with Archbishop Lefebvre at the head," explained the Rev. Clarence Kelly of East Meadow, N.Y., who described himself as the "superior of the society in this country."

The society has four centers in this country: its Long Island headquarters, a makeshift seminary in Michigan, Queen of Angels Church in a Houston, Tex., suburb that the archbishop consecrated earlier this month, and another church in San Jose, Calif.

The funds come entirely from their followers, Kelly says, whom he describes as "mostly middle class - no really big contributors." There is of course, no support from either the Vatican or diocesan budgets.

The society's 11 U.S. priests, all but three trained at Lefebvre's rebel seminary at Econe, Switzerland, travel out from the four centers to scattered congregations to celebrate the ancient Latin Mass and teach doctrines no longer accepted by the established church.

"But there are many other priests who are doing the same thing," Kelly insisted.

No one knows how many closet traditionalists the Catholic Church harbors today. One of the many changes wrought by Vatican Council II 14 years ago was the decision to replace the 400-year-old Tridentine Latin Mass with a liturgy in the day-to-day language of the faithful.

Changes in religious rites are always difficult. Hearing the services in the same language with which one buys potatoes seemed, for many, a sacrilege.

These disgruntled Catholics have sought like-minded priests willing to defy the church by continuing to say the Latin liturgy in clandestine - and sometimes defiantly open - rites.

Some complaining bitterly of the "guitars" and "banners" of the contemporary Masses in their local parishes stay home and satisfy their consciences, if not church requirements which oblige Catholics to attend Mass weekly, by listening to the banned Latin Mass on tape cassettes.

The latter are available from the Rev. Gommar A. De Pauw, onetime dean and professor of canon law at Mount St. Mary's Seminary in Emmitsburg, Md., and founder of his own Catholic Traditionalist Movement, which predates the Lefebvrists, whom he criticizes sharply. De Pauw's Latin masses, taped, as he advertizes "at a real alter" in the Ave Maria Chapel he built in Westbury. Long Island, are also heard on some radio stations in the Sunday morning religious hours.

There are little pockets of traditionalists in virtually every diocese of the country. But there is little cooperation among them. Mary follow renegade priests, who, like Lefebvre, function in defiance of church authorities.

Typical is the Rev. Francis Fenton. In 1963, he took an open-ended leave of absence to hit the lecture circuit for the John Birch Society. In 1973, still technically on leave from his diocese, he returned and founded the Orthodox Roman Catholic Movement with his own traditionalist parish near Startford, Conn., withing diocesan boundaries but totally without authorization of Bishop Walter Curtis.

The movement has prospered and today, he says, he has six additional "chapels" in Colorado, New York, Louisiana, Montana and South Dakota. Fenton, who still continues on the Birch Society lecture circuit, does not cooperate with De Pauw and the Lefebvrists.

Under church law, priests who persist in using the Tridentine Mass. except in special circumstances, can be suspended.

But bishops are reluctant to take harsh measures against such priests for fear the action would draw sympathy and attention to their cause.

In the case of Fenton, a diocesan spokesman explained that Bishop Curtis "has not suspened him, but considers him a disobedient priest."

Fenton says, "Technically I am in good standing, amazingly."

If there has been a lack of cooperation among traditionalist groups in the past, there are things they have in common - unhappiness over the sweeping chuch reforms of the last decade, nostalgia for the past and deep emotional attachment to the Latin Mass.

For the followers of Archbishop Lefebvre, the issue goes far beyond the Latin Mass. In founding his Society of St. Pius X, the rebel archbishop chose as its patron the turn-of-the-certury Pope who devoted much of his 11-year pontificate to battling "modernism" in the church.

The Lefebvrists reject everything that came out of the worldwide conclave of bishops known as Vatican II, called by Pope John XIII. "Vatican II has to be jected," said Kelly, their U.S. spokesman.

They attack the liturgy in use for the past seven years as "sacrilegious" not only because of the missing Latin, but also because the words of consecration were changed to reflect a new, less exclusivist theology. The new liturgy asserts that Christ died for "all" men rather than "for many," as stated in the older liturgy.

Kelly also objected to the new Mass because "a Protestant could say it without violating his conscience."

The old Tridentine Mass expressed "reverence for the Pope," he said. "Everything about it is completely Catholic. In the new Mass, that's not true. In the new Mass, a non-Catholican say it without violating his conscience. The Mass should be so thoroughly and completely Catholic that only a Catholic can say it."

The Rev. Donald Sanborn, who heads the traditionalist seminary at Armada, Mich., holds that the Mass, as said in the Catholic Church today, "is of dubious value" because of the changes. "It's at least dubious if not invalid . . . According to canon law, if it's dubious, then it's a sacrilege to use it."

The Lefebvrists reject Vatican II's view of Protestant churches as fellow-Christian bodies.

"We say the Protestant church is not a church at all, but a human institution created by men who have revolted against the mystical body of Christ," said Kelly.

"We say this not to insult people but because it's true," Kelly continued. "Only the Catholic Church is the true church. Only the Catholic Church was established by Jesus Christ. It's our obligation not to join these groups together but to convert members of these groups to the Catholic faith. It's our obligation to evangelize, to say that the Catholic Church is the true church, that you should reject this man-made religion [the Protestant churches] and embrace the true faith."

Vatican II's repudiation of the section of the Good Friday liturgy that asserts the Jews are responsible for Christ's death is also rejected by the Lefebvrists, who still pray on Good Friday for "the perfidious Jew." The allusion to Jews as Christ-killers in the ancient liturgy was expunged by the church after Vatican II. But for traditionalists, Kelly said, "the whole point is to pray for those who are not members of the Catholic Church.

In the view of Kelly and many traditionalists, Vatican II is the cause of a host of church problems, ranging from the defection of large numbers of priests and nuns to the rocketing divorce rate to the "socialist" positions of the bishops.

He charged that American bishops, by permitting academic freedom in the church's seminaries, are undermining moral teaching. "What they say with their mouths and what they do in the administration of their dioceses are two different things," Kelly charged.

He also faulted the American bishops for "becoming increasingly oriented toward socialism" in their pronouncements on society's problems. "They are calling on government agencies to solve problems which might be solved by private works of charity," he said.

Sanborn, head of the traditional seminary in Michigan, justified disobedience because, he said, the Pope "is caught up in the heresy of modernism in the church."

The situation, he said, is like that of a child ordered by a parent to steal. "Catholic teaching tells us that the child may disobey his parent when the parent tells him to do something he knows to be wrong," Sanborn said.

According to Bishop-elect Thomas Kelly, general scretary of the National Conference of Catholic Bishops, American Catholic bishops will take their cue from the Vatican on how to deal with the Lefebvrists in this country.

"They have tended to deal with traditionalist priests in a pastoral manner in the past," he said.

Typical is a recent pastoral letter issued by Boston's Humberto Cardinal Medeiros. The Boston prelate lumped the conservative archbishop together with liberal thelogogian-authors of a recently published human sexuality study that strays from official church teaching.

Cardinal Medeiors, in a 2,400-word statement, warns the faithful eqully against the conservative Scylla on the right and the liberal Charybdis on the left.

However dogmatic it may appear to outsiders, the Catholic Church encompasses a wide range of viewpoints. As Archbishop Lefebvre continues his attacks on church leaders as well as doctrine, demands will increase from outraged Catholics that Pope Paul excommunicate him.

That is a step the pontiff is clearly reluctant to take. Time and again he has made clear that the job of a Pope is to hold the church together not to redline troublesome parts of it. And excommunication, as Bishop-elect Kelly points out, "is a terribly serious business."

Besides, the Vatican took that line with a troublesome German monk - Martin Luther - some 400 years ago. It didn't turn out to be a very good idea.