Having endured their toughest year since World War II, Filipinos are marking a relatively austere Christmas amid official predictions of progress toward economic recovery next year but continuing political uncertainty.

President Ferdinand Marcos, who appears to have recovered somewhat from an illness that isolated him for two weeks last month, declared in an address to the nation broadcast tonight that this was "a time of resurgent faith in ourselves and in our nation."

While the Philippine gross national product shrank 5.5 percent this year in the first decline since the war, according to the latest government figures, Marcos and other officials have cited brighter economic prospects in 1985 because of an improving balance of trade and a just-concluded agreement with the so-called Paris Club of industrialized nations to restructure $1.1 billion in loans from foreign governments and their institutions.

The Dec. 20 agreement followed a long-negotiated accord with the International Monetary Fund on an economic recovery package providing for $615 million in standby credits and paving the way for restructuring of the country's $25.6 billion foreign debt.

Yet, inflation is still running at more than 50 percent, the highest rate in Asia, and hundreds of thousands of people have been affected by layoffs and cost-cutting measures in numerous businesses and industries in a year that has seen industrial output fall by nearly 9 percent. Economists predict that thousands more will lose their jobs early next year before economic rescue measures take effect.

In a gloomy Christmas message aired today, Cardinal Jaime Sin, the leader of the Roman Catholic Church in the Philippines, said that Christmas carols in this predominantly Christian country this year "have a hollow ring" and expressions of good tidings "a mocking tone." He called for greater attention to the needs of the poor, as better off Filipinos busied themselves with their usual holiday festivities and crowded Manila's stores in a gift-buying spree.

In his own Christmas message, President Marcos noted that the nearly year and a half of "crisis and adversity" since the assassination of opposition leader Benigno Aquino Jr. had left "a sense of anguish and deprivation" that could not be quickly overcome. He said, however, that no one "can possibly doubt now the real progress we are making toward recovery and the measure of stability we have attained."

While he appeared hesitant at times in his speech, Marcos' voice sounded stronger and he looked fitter than in recent weeks.

Concerns over the Philippines'stability nevertheless have persisted amid continuing doubts about the true state of the 67-year-old president's health, a growing communist insurgency in the provinces and disarray in the ranks of the legal political opposition. The health rumors still circulate despite Marcos' recent exposure of part of his torso in a Cabinet meeting -- a display that opponents dubbed "the presidential striptease" -- to deflate reports that he had undergone surgery. Many Marcos foes say they remain unconvinced.

Fueling the rumors have been increased public appearances lately by the president's wife, Imelda Marcos, who, some opposition figures feel, is being groomed again to succeed her husband.

One opposition member of parliament, Homobono Adaza, expressed concern in a speech Saturday that unless the opposition unites behind a single presidential candidate soon, a Marcos strategy to keep his 19-year-old presidency in the family might succeed.

Adaza also criticized a three-member opposition "convenor group" formed to speed up the selection of a presidential candidate in the event Marcos leaves office before his term expires in 1987 and an election to replace him is scheduled within 60 days of the vacancy. Adaza said the group -- composed of former senator Lorenzo Tanada, 86, the opposition's patriarch; Corazon Aquino, the widow of the slain opposition leader and Jaime Ongpin, a prominent businessman -- was "no less a junta, no less authoritarian" than the Marcos administration.

The convenor group met last week with some of the 11 potential presidential candidates it has chosen or their representatives. They agreed on a set of general principles based on "resistance to authoritarianism" and support for a "pluralistic society." The 11 include former senators Jose Diokno, Salvador Laurel, Teofisto Guingona, Raul Manglapus, Ambrosio Padilla and Jovita Salonga; present members of parliament Eva Estrada Kalaw, Ramon Mitra and Aquilino Pimentel; former presidential executive secretary Rafael Salas and Agapito Aquino, the younger brother of the assassinated opposition leader.

How these candidates were chosen has excited considerable criticism in opposition circles, especially by those who felt left out.

One of those on the list, Kalaw, also has expressed reservations about the convenor group's "fast-track" system of choosing a candidate in the event of a sudden presidential vacancy on the grounds that it appeared to be "undemocratic" and would bypass a national opposition political convention.

She criticized the "thesis that a few, no matter how well meaning, can impose their will upon the people by the strength of their station."