After attending a mass in Seoul's Myongdong Cathedral one morning last week, 60 South Korean Roman Catholic priests turned in special white cards to church officials. Each card was signed and bore an affirmation of personal support for a campaign to amend the South Korean constitution to require the direct election of the president.

With acts like this, South Korea's Roman Catholic Church and many Protestant churches are becoming increasingly involved in the battle to remove the six-year-old government of President Chun Doo Hwan.

Cardinal Stephen Kim, leader of South Korea's Catholics, has endorsed the constitutional revision drive, which has become a vehicle for expressing opposition to Chun. So has the Korean National Council of Churches, which links six Protestant denominations.

Inevitably, people here are drawing parallels with the Philippines, where activism by the Catholic Church was instrumental in bringing down President Ferdinand Marcos. But there are deep differences, notably that Christians are a minority here, making up only about a quarter of the population, and are divided by factionalism.

Still, many church leaders command respect among people of other religions (including Buddhism, the predominant faith here), and are giving a new moral authority to the fight against Chun.

H.C. Hyun, a National Assembly member of Chun's Democratic Justice Party and a former deputy director of the Korean Central Intelligence Agency, calls Cardinal Kim "one of the very influential intellectuals in Korea." Hyun said that "the fact he has supported the opposition camp's position will have a very great impact."

Both Protestant and Catholic leaders deny that they are being partisan, due both to fears of provoking government response and to their own religious dogma. But when they talk of putting a nonviolent end to "dictatorship," no one needs to ask whose they have in mind.

"The fundamental reason for our actions is the mission of God," said the Rev. Augustine Ham, a priest who was jailed for political offenses twice in the 1970s and is today chief spokesman for the Korean Catholic Church. "When the situation requires, the church must speak up."

Korea and the Philippines are the only East Asian countries where Christianity has taken root firmly. Catholicism, which claims about 2 million followers among the 40 million South Koreans, was introduced here in 1784. Protestantism arrived a century later. It now has about 8 million adherents and covers just about every major denomination, including Presbyterians (the largest), Methodists and Baptists. South Korean skylines are punctuated with church spires.

Korean Christians traditionally have been at the forefront of movements for political change. The religion's break with concepts of strictly hierarchical society in the country's Confucian heritage is often mentioned to explain this role.

Fifteen of the 33 signers of a failed declaration of independence from Japan's colonial rule in 1919 were Christians. Today, South Korea's two best-known opposition leaders, Kim Dae Jung and Kim Young Sam, are both Christians.

Christians resisted the Communists' consolidation of power in the north after 1945, and today their faith has been all but eradicated there. But in the south, it is expanding fast -- especially Catholicism, which claims a 5 percent to 7 percent growth rate per year. The pace has picked up noticeably since Pope John Paul II's visit here for the church's bicentennial celebrations in 1984.

The Seoul headquarters of the National Council of Churches long has been a clearinghouse for dissidents and is watched closely by the authorities. The council works with families of political prisoners, which it says now number about 800, the highest figure since Chun took power in a military coup six years ago.

Like its affiliated organization, the World Council of Churches, it is often criticized by conservative Christians as turning too far from spiritual things and backing revolution.

Last week, a group called the Council of Christian Leaders of Korea, which claims the allegiance of about 10,000 Protestant congregations, met at a Seoul hotel and denounced the growing politicization of the church. "Religious people taking part in politics is undesirable," said the Rev. Choe Hun, a Presbyterian minister who heads the group.

Assemblyman Hyun of the ruling party suggests that by speaking out about politics, religious leaders in the long run are decreasing their effectiveness.

There is no sign to date that the government is working directly with conservative Christians, but the National Council of Churches says it expects that to happen. The Rev. Carl McIntyre, the American fundamentalist evangelist, recently was seen on a government TV news program here condemning the Korean National Council of Churches' actions.

The Catholic Church here has far fewer problems with internal unity, though officials concede there is some dissent. So far, about 350 priests are known to have signed.

The Catholic hierarchy was active in resisting Park Chung Hee, South Korea's previous president in the 1970s, but had been fairly quiet since Chun's ascension to power. That was broken earlier this month when Cardinal Kim, citing dangers from "selfish desires, prejudices and dictatorship," called for rapid revision of the constitution.

Christian leaders work closely with the New Korea Democratic Party, the main opposition group in the National Assembly. For instance, the National Council of Churches passes names of newly arrested political prisoners to the party so their names can be raised at legislative sessions.

Still, the churches are careful not to appear to get too close to the opposition and always speak of reconciliation as the best solution. They appear to fear that a united front of church, party and student groups could frighten the government and invite a harsh response.

On comparisons with the Philippines, church leaders acknowledge differences but nonetheless talk of inspiration. "Since Catholics played an important role there," Ham said, "many people expect one here, too. Our mission has become even more heavy than before."