The more than 2 million Roman Catholic men and women in military service and Veteran's Administration institutions worldwide got their own archbishop and their own Military Archdiocese this week.

Government, military and religious leaders gathered at the National Shrine of the Immaculate Conception Tuesday for the installation of Archbishop Joseph T. Ryan as head of the new jurisdiction, to be headquartered at Catholic University.

The colorful ceremonies marked a decision by the Vatican earlier this year to give independence and a new visibility to the ministry to Catholics in the armed forces, elevating it from the status of an adjunct of the Archdiocese of New York to independent archdiocesan status.

Cardinal-elect John J. O'Connor of New York, preaching at the installation service, said the move demonstrated that Pope John Paul II recognizes that the "territorial dimension and enormous demands" of service to military personnel "are such that it is truly right and just that you have an archdiocese to be called your own."

Secretary of Defense Caspar Weinberger read a message from President Reagan declaring "my conviction that spiritual strength is vital to our national survival."

The existence of a special office for oversight of Catholics in the military dates back to World War I when Bishop Patrick J. Hayes, then an auxiliary of the New York archdiocese, was designated by the Vatican as military bishop for the Army and Navy chaplains in the United States.

The church's ministry to the military received its highest visibility under Cardinal Francis J. Spellman, who, during World War II and the Korean War, often visited troops on distant battle fronts.

In more recent years, the worldwide flock of the military vicariate has been expanded to include Catholics in diplomatic and other government missions overseas, as well as military. The 2.2 million Catholics in these categories make the military archdiocese third in the nation -- after Los Angeles and Chicago -- in Catholic population size.

There are now about 1,000 priest-chaplains serving on active duty.

In his homily, O'Connor, who headed the military vicariate for four years under Cardinal Terence Cooke and served 27 years as a chaplain in the Navy, cited the U.S. Catholic bishops' 1983 pastoral letter on nuclear war, which he helped draft. "No one is more sensitive to the moral dimension of the pastoral letter of the bishops than are those in uniform of every religious persuasion," he said.