A group of Roman Catholics who believe that dedication and prayer to Mary, the mother of Christ, is the only way to "convert Russia and prevent our annihilation in a third World War," is showing a dramatic increase in area membership.

An international organization founded in 1947, the Blue Army of Our Lady of Fatima has been recognized officially by recent popes as the apostolate of the message of Our Lady of Fatima, named for visions of Mary that are said to have occurred in Fatima, Portugal, periodically throughout 1917, the same year, observers point out, as the Bolshevik Revolution.

Claiming 20 million members worldwide and 5 million nationally, the Blue Army is dedicated to carrying out instructions from Mary to three children in six separate apparitions.

Members of the group, which went pretty much unnoticed in the Washington area for more than a decade, believe that war, and World War II in particular, is the result of personal sin, but that war can be prevented by making reparation to God in the form of personal sacrifice and prayer, particularly by reciting the rosary.

In the last two months, the Blue Army has picked up 25 new members as the result of making a presentation to one group of Catholics who had never heard of it before, and now numbers more than 500.

The group plans to make presentations to all 127 parishes in the Washington Archdiocese. "We hope to have Blue Army units in all the parishes by May 13, the 61st anniversary of the vision," said Edward Albrecht, president of the Washington Archidiocese's Blue Army Center. The only official parish unit is at St. Mark's parish in Hyattsville, but more than 10 other area parishes hold a monthly Blue Army prayer service.

"We just showed them two films and the pledge cards and scapulars just disappeared," said Connie DiVito of the 25 new members at a recent meeting of the group.

Blue Army members pledge to wear a green scapular that symbolizes their devotion to Mary, say a part of the rosary daily, and "offer all our actions during the day, a consecration of ourselves to fulfill the conditions of the message of the apparition," as the vision instructed.

The official chronicle of the visions was written by the sole survivor among the three children at Fatima, now a cloistered Carmelite nun popularly known as Sister Lucia, and warns in part: "If my wishes (for prayer and consecration) are fulfilled, Russia will be converted, and there will be peace; if not, then Russia will spread her errors throughout the world, bringing new wars and persecutions of the church . . ."

The first film shown to prospective members is "The Price of Peace and Freedom," a 1976 production of the private American Security Council Education Foundation, and is a comparison of U.S. and Soviet offensive and defensive strategic weapons.

It consists of a half-hour of film clips of heavy Russian weapons, interspersed with comments by American military officials.

A typical filmed comment is that of former Secretary of the Navy William J. Middendorf II: "Today we face a Soviet threat far greater than any other threat this nation has ever faced in its 200 years of existence."

The second film, "Century of Blood," was produced by Robert Bergin, a 63-year-old Australian who has been active in promoting the Blue Army worldwide for 20 years.

Bergin, who is helping the Blue Army increase its membership while he is here in Washington on a visit, explained the message of his film during an interview:

"I traced the incidence of wars in the last century, then I showed the cemeteries, then many battle scenes, then Iwo Jima. Then I posed the question, 'How can we stop this continuing slaughter?' Is there any hope? Then I reminded the people that a message has been given from heaven to tell us exactly how the war could be stopped. I also remind them that the message indicated that there is no other way."

"The purpose of showing these films is not to encourage the military," said Jane Prahinski, who is visiting area parishes to encourage those under 18 to become Blue Army cadets. "In the apparition there was the prediction that there would be the annihilation of nations. We do want to show the possibility of military warfare. There might be an atomic warfare as punishment."

Bergin, when asked what could account for what he sees as a resurgence of American interest in the Blue Army after a waning for many years said, "Maybe Americans are beginning to think the unthinkable, particularly at the grass roots, because those at the top appear to be numb. The mutually assured destruction is the only thing the administration can promise us. That is, after we're dead, the retaliatory strike will destroy the Russians. That isn't much to ask for after 150 American cities have been destroyed, is it?"