The Roman Catholic Jesuit order expelled Nicaragua's education minister, the Rev. Fernando Cardenal, today in the church's most severe action so far against priests who hold positions in the government here.

The religious order announced at its Rome headquarters that Cardenal had received a letter on Dec. 4 informing him that he had been expelled for disobeying orders to resign his position as a government minister. Although he remains a priest outside the Jesuit order, the statement issued in Rome said Cardenal was suspended from carrying out priestly functions until he received permission from a local bishop to do so.

Cardenal released a 19-page statement this afternoon saying that he would remain in his post and that he had "an objection of conscience" to obeying the order to step down.

"Sincerely, I consider before God that I would commit a grave sin if I abandoned in the present circumstances my priestly option for the poor, which currently takes its concrete form in Nicaragua in my work in the Popular Sandinista Revolution," the statement, issued in the form of a printed booklet titled "Letter to My Friends," said.

Cardenal said that the church was pressuring him to resign for political reasons rather than theological ones, and he specifically blamed Pope John Paul II for preventing him from staying on as a government minister.

"From here we perceive that the Vatican's policy toward Nicaragua coincides with that of President Reagan," Cardenal's statement said. "It seeks to delegitimize the revolutionary process with our withdrawal."

The Vatican has stepped up pressure this year on Cardenal and three other priests in senior government positions since a new Roman Catholic code of canon law took effect in November 1983 prohibiting priests from holding public office. The four also have come under pressure from Nicaraguan bishops who also are at odds with the Sandinista government over a variety of other issues.

"There's no doubt that the pressure is on, both from the Vatican and from individual bishops here," a government source said.

The other three priests holding public office are Foreign Minister Miguel D'Escoto, who belongs to the Maryknoll order; Culture Minister Ernesto Cardenal, a member of the Trappist order and Fernando's brother; and the ambassador to the Organization of American States, Edgard Parrales, who belongs to no religious order.

It was uncertain, however, whether the Jesuits' action against Fernando Cardenal meant that similar steps would be taken soon against the other three priests. That decision was in the hands of their own religious orders -- in the cases of D'Escoto and Ernesto Cardenal -- and of the Nicaraguan bishops responsible for them.

Fernando Cardenal always has been considered to be in a particularly vulnerable position because the bishop who is his superior is Msgr. Pablo Antonio Vega, president of the Nicaraguan Bishops' Conference and the church's most outspoken critic of the Sandinista government. D'Escoto and Parrales, by contrast, are responsible to a bishop in the northern city of Esteli who is considered to be more sympathetic to the priests.

In addition, the Jesuits were viewed as particularly sensitive to Cardenal's position because the order, formally known as the Society of Jesus, has drawn criticism from the pope for its involvement in political causes.

The Vatican issued a statement on Aug. 10 warning the priests to step down from their posts, and there were reports at that time that bishops here had fixed Aug. 31 as a deadline for the resignations. That date came and went without action, however, and a Nicaraguan government delegation visited the Vatican in September in part to seek an agreement on the issue.

Nicaragua's bishops first began pressuring the priests to resign in 1980, after Pope John Paul II made it known that he opposed allowing priests to serve in governments anywhere. A compromise was arranged in 1981 in which the priests kept their positions but agreed not to perform the sacraments or to wear priestly dress as long as they held public office.

There have been persistent rumors in the past month here that one or more of the priests was to resign as part of a government shake-up expected to take place either just before or just after the inauguration of Daniel Ortega as president on Jan. 10. Ortega, who currently is chief of state in his capacity as coordinator of the governing three-man junta, is to assume the new title of president, which he won during national elections here on Nov. 4.

Fernando Cardenal took office as education minister in July, when he replaced Carlos Tunnermann after Tunnermann was named ambassador to the United States. At the time the Jesuit order formally advised him against taking the post.

Before taking his post as minister, Cardenal had worked for the government since the 1979 revolution, first in the national literacy program and later with the Sandinista youth movement.