It was the second Sabbath since Brig. Gen. Efrain Rios Montt, a born-again Christian, had emerged as head of Guatemala's new ruling military junta, and the American elders of his evangelical church had come to town to hail him as a new savior.

As he stood under the yellow and green awning of a revival tent with his hands held skyward and the congregation chanting, "hallelujah," Rios Montt's sudden ascension to power two weeks ago was described to him from the pulpit as "a miracle."

"God has raised up a leader of this nation," said Jim Durkin, the founder and presiding elder of Gospel Outreach, an evangelical movement based in Eureka, Calif., "a man of destiny, a man of God." Under Rios Montt's leadership, Durkin said, Guatemala would be lifted above "all the nations of the world" by its example of "righteousness and justice."

In a land dominated by the Catholicism of the Spanish conquistadores, U.S. diplomats and Guatemalans alike are still puzzling over the brigadier general and one-time presidential candidate who returned to Guatemala from exile four years ago to renounce his Catholicism and become a lay evangelical preacher for the Church of the Complete Word, a local arm of Durkin's Gospel Outreach.

"Frankly nobody knows just what to make of him," one senior European diplomat here said, noting that the U.S. Embassy has remained oddly standoffish about the new president, waiting almost a week before meeting officially with him. "He is so different that nobody has really been able to figure him out."

The religious fervor of his speeches and the constant references to "God, my master, my king," with which he laces his pronouncements have already caused many uneasy Guatemalans to dub him "Ayatollah Rios Montt."

Stern and mystical, Rios Montt defies the corrupt and authoritarian stereotype of the Guatemalan general, personified in the man he replaced following a March 25 coup, Gen. Romeo Lucas Garcia. Guatemala's new ruler is an up-from-the-ranks military officer who rose to head the country's military academy as well as to become armed forces chief of staff. In 1974 he was tapped to run as a moderate candidate for president with the support of the Christian Democratic Party against the government's handpicked general.

Many believe that if the government had allowed the votes to be counted, Rios Montt would have been elected president. The government did not and instead declared its own candidate, Gen. Kjell Eugenio Laugerud, the winner and sent Rios Montt into exile as its military attache in Madrid.

Returning to Guatemala in 1978, after Lucas Garcia had replaced Laugerud, Rios Montt, whose brother Mario is the Catholic bishop of Escuintla, soon became affiliated with the California-led Church of the Complete Word, or El Verbo, as it is known in Guatemala. The evangelical church had been established after Guatemala's devastating earthquake in 1976 by a group of 25 missionaries who came from California to spread God's word and to help build houses for the homeless.

So total was Rios Montt's conversion 3 1/2 years ago that he virtually dropped out of sight in Guatemala City to devote himself to running his church's grammar school and Bible classes as well as serving as the leader of the one of the sect's 12 "churches in the home."

Rios Montt had been out of contact with his old military colleagues and the politicians who had supported him in 1974, and it is apparent that the young officers who called him to be the figurehead in their coup had not the vaguest idea of the spiritual transformation of their former garrison commander and military academy director.

Called by radio broadcasts to come forward to lead the coup, Rios Montt, who had been heading a parent-teacher meeting when word of the coup reached him, first met with fellow elders of his church, at least several of them Americans, to pray and ask for the Lord's guidance.

"We prayed, and when we felt a sense of peace, we agreed that it was God's will and that he should go," recalled Elder Richard Funnell, formerly of Belair, Md.

Within hours after joining the young coup-makers, who had advocated quick new elections that it is generally conceded would have favored the extreme rightist National Liberation Movement, Rios Montt had taken control. The young officers found themselves little more than "advisers" to Rios Montt's own senior military triumvirate.

Rios Montt has said nothing about elections nor a return to civilian rule. Instead he has launched on a cleanup of the government that has all the earmarks of a spiritual crusade against the abuses of power and corruption that long have typified Guatemalan officialdom.

At least 10 former ministers and public works officials under Lucas Garcia have been charged with corruption. Dozens more are being sought. All bureaucrats have been forbidden to leave the country until they can be investigated.

Lucas Garcia; his brother Benedicto, the former chief of staff; the unpopular chief of police and other generals, including Gen. Angel Anibal Guevara, Lucas Garcia's handpicked successor, are still under house arrest pending investigation.

Rios Montt has ordered a curb on the activities of the secret police, the judiciales, a notorious group of people who U.S. officials say often work as "death squads" to eliminate critics, real or imagined, of the government and whose activities gave the government one of the worst human rights reputations in Latin America.

Even critics of Rios Montt, worried initially about his unpredictability, as well as his staying power, have been heartened by his seeming determination to restore morality and justice to Guatemalan government.

"He is no wizard of Oz, and his ways of explaining things are sometimes odd," said one moderate Christian Democratic politician, "but so far it is hard to fault him on what he has done. He seems to be trying to clean up the government and our tarnished world image. We can hardly argue with that."

In the church under the tent yesterday, surrounded by five U.S. evangelists who came from California with Durkin, Rios Montt was subdued as he listened to his church elders extol the miracle of his rise to power.

"Oh Lord, let this man and the nation of Guatemala be a mighty light, a beacon to draw wanderers to God," Durkin said in an invocation. "Let this be a land where the songs of Zion resound, where prosperity thrives and a land which will serve as an example of the Lord's presence on earth."

"There's not an army in the world, not all the power that the devil has at his disposal, that can stop you now from going to your destiny," Durkin told Rios Montt and his congregation of about 400 worshipers. "We are all God's children; we are filled with His grace and charged with spreading His word."