A surprisingly small, subdued crowd gathered in the capital of this western-most U.S. possession this morning to hear Pope John Paul II exhort Roman Catholics at an open-air mass to spread their faith.

The pope arrived late yesterday, stayed 18 hours and departed for a three-day visit to Japan. He was welcomed here by Mike Mansfield, the U.S. ambassador to Japan and President Reagan's personal representative, as well as by modest crowds.

About 15,000 islanders clustered in what were once the palace grounds of Spanish colonialists to receive communion from the 60-year-old pontiff today. Local officials had estimated that nearly 60,000 of the island's 105,000 residents would pack Agana for the mass.

"Evangelization is the core of the church's activity in the world. Here lies her calling, here begins her work, here is her greatest challenge," the pope told a crowd that Vatican officials said was among the most subdued assemblies ever ot greet the pope. Local officials blamed the low turnout on tight security.

Islanders milling through the crowd said advance reports of tight security and large crowd predictions kept many from trying to see the pope. They also said that many stayed home to watch live TV coverage.

Yesterday, about 10,000 islanders waited at the airport, along the motorcade route and at the Dulce Nombre de Maria Cathedral to greet the pope after his grueling six-day visit in the Philippines.

After kissing the ground, John Paul told the crowd waiting at the airport, "with a heartful of gratitude, I set foot on your native soil and kiss it as an expression of my respect and reverence for the people of this territory."

The pope drew cheers when he switched from English into Chamorro -- Guam's language that is a mix of Malay, Spanish and Tagalog -- three times during his 10-minute airport speech.

Guam, creded to the United States by Spain in 1898, was the site of a major World War II battle and is a bastion of U.S. military strength in the Western Pacific. One third of the land and about 20 percent of the population is military.

Special correspondent Abby Tan reported from Manila on the windup of the pope's stay in the Philippines:

John Paul, in his final pastoral act here Sunday, lent his support to Filipino tribal groups resisting government development efforts as a threat to their culture.

He stated the church's continued support of tribal efforts to preserve their culture and their right to be consulted. President Ferdinand Marcos sat listening as the 60-year-old pontiff, showing signs of fatigue, celebrated mass in Baguio city, 150 miles north of here, with the mountain tribes who inhabit the area.

Only some of the tribes have been converted to Christianity but the pontiff's presence was a symbol of the support they had received from the church in their resistance to government efforts to evict them from their land.

Tribal groups make up 12 percent of the Philippines' 48 million population.

Encroachment on their ancestral lands has caused friction in recent years.

No other issue triggered more bitterness and bloodshed than the government plans to build a multimillion dollar Chico dam in the northern area visited by the pope.