Just six weeks after his return from Africa, Pope John Paul II arrives Monday in Brasilia, the start of a 12-day, 10,000-mile journey through the world's most populous Roman Catholic country.

He arrives at a time when relations between the military government here and the "progressive" wing of Brazil's militant church are particularly strained because of the church's defiant support for a metalworkers' strike in April and for a radical land reform program.

The pope's visit also comes at a time when the Brazilian church itself is split among progressive, moderate and conservative bishops. They are engaged in an open and sometimes hostile battle for control of the church, which has become increasingly radicalized over the past decade. Militant Catholic clergymen have been fighting for human rights and the welfare of tens of millions of poor, illiterate Brazilians who have not shared in the country's economic progress.

A measure of the change within the Brazilian church is its attitude toward the military. In 1964, when the armed forces seized control of the government, the church supported the coup. Today, the church is considered one of the government's most critical and powerful opponents and possibly the most militant in the hemisphere.

The head of the military government, President Joao Figueiredo, in April publicly accused one of the church's leaders, Cardinal Dom Paulo Evaristo Arns of Sao Paulo of encouraging and aiding the metalworkers' strike. The government had declared the strike illegal and attempted to suppress it by closing the union headquarters and arresting the union leadership.

In response, the church opened its buildings for union meetings and collected food, clothing and money to assist the striking workers. After 41 days, however, the strike collapsed, leaving bitterness on both sides.

Earlier this month, Sen. Jarbas Passarinho, a leader of the progovernment Social Democratic Party, charged that portions of the church were preaching the government's overthrow and, as a result, were "receiving the continued praise of the communists."

Since 1977 the church hierarchy here has issued a series of policy statements openly critical of the Brazilian political and economic model, which combines repression of unions and leftist political movements with laissez-faire capitalism dominated by multinational corporations.

The church has accused these corporations -- and the government itself -- of exploiting workers, who earn between $90 and $200 a month for 8 hours of work a week in Sao Paulo's large factories. Workers complain they have no job security and are regularly dismissed without cause, a central issue in the recent metalworkers' strike.

The church also has accused the government of neglecting social needs. At least 30 percent of the population is illiterate, and an estimated 30 million people live in poverty. The infant mortality rate is 110 per 1,000 births, 10 million people are infected with deadly "chagas" worms, and 40 million people, the majority of them children, are believed to be undernourished.

"It is often said that Brazil is one big hospital without hospitals'," according to one priest who works among the poor in teeming metropolis of Sao Paulo. An estimated 900,000 people live in wood and cardboard huts without heat, electricity, running water, sewers, medical attention or enough food to fulfill basic nutrional needs.

As bad as the poverty is in these shantytowns, known as favelas, thousands of Brazilians continue each year to migrate to Sao Paulo from the northeast of Brazil, a largely agricultural area that regularly suffers from droughts or floods. There the poverty is even worse.

It is against this background that Pope John Paul II arrives in Brazil. Although on past trips to Africa and Mexico he has weighed his words carefully when talking about the church's ministry to the poor, observers believe that he may be unable to avoid taking a stand during his 12 days in Brazil.

John Paul II is scheduled to spend more time here than any pope in modern times has spent in any one country outside Italy. This is only the most obvious sign of the importance he apparently attaches to Brazil, a vast and varied country that occupies half of the South American Continent and is home to more than 120 million people, about 95 percent of them baptized Catholics.

Besides celebrating outdoor masses in the country's major cities, the pope is to visit Brazil most important religious shrine at Aparecida. He is scheduled to spend 45 minutes with Figueiredo in the capital, Brasilia, visit a leper colony in Belem, tour a favela in Rio de Janeiro and meet with workers in Sao Paulo. Ovservers believe he will have something to say about the macumba voodoo cults so popular with Brazilians of all classes when he visits Salvador de Bahia in Northeastern Brazil.

However, the church hierarchy, the Brazilian government and the Latin American church in general are most anticipating a papal pronouncement about the direction the church here has taken on social and economic issues affecting the poor.

In the past, the pope has said he opposes the church's becoming involved in partisan political activities. The government here, its supporters and other military rulers in Latin America have interpreted this as a prohibition against church activity in human rights movements and strikes.

But the pope has also said it is the church's duty to support social movements and activities that help secure economic and social justice for the poor. Progressive church leaders and priests here interpret this as support for their "theology of liberation" and their efforts on behalf of the poor in Brazil.

The line the pope is trying to draw is not always clear, expecially in Latin America. Here the church is fighting against injustice as it sees it, which often means fighting against right-wing military governments without necessarily supporting or worrying too much about the political parties or ideologies that might replace them.