Pope John Paul II sternly upheld celibacy for Roman Catholic priests today and warned that he would make it much more difficult for priests to get Vatican approval to leave the priesthood.

While the pope's opposition to marriage for priests, contained in a letter addressed to clergy around the world, reaffirmed the position of Pope Paul VI and the Second Vatican Council on the issue, observers noted that it was his second strong statement on the sensitive matter so far in his brief reign.

Pope John Paul's hard-line position against priests returning to lay life is in sharp contrast to the policies of Paul VI, who, according to some reports, allowed more than 30,000 priests to be dispensed from their vows and leave the priesthood.

The permission to leave the priesthood yet remain in good standing as lay Catholics had been considered by many Roman Catholics as an escape valve that prevented severe individual hardships.

Since his election last October, John Paul has blocked most requests for dispensations from priestly vows and the Vatican has indicated that only exceptional cases will be considered.

In his letter today, the pope quoted St. Paul as saying,"There is nothing I cannot master with the help of the one who gives me strength."

The Pope said, "One must think of all these things especially at moments of crisis and not have recourse to a dispensation. . . The priesthood cannot be renounced because of the difficulties that we meet and the sacrifices asked of us."

John Paul critized attempts to "secularize" the priesthood and "bring it up to date."

Any view that celibacy is forced on those who become priests "is the result of a misunderstanding, if not of downright bad faith," he said.

A priest decides upon a life of celibacy "only after he has reached the firm conviction that Christ is giving him this 'gift' for the good of the church and the service of others," he said.

Catholics, the pope said, "have the right to expect from us priests and pastors, good example and the witness of fidelity to one's vocation until death.

The pope acknowledged that there is a worldwide shortage of priests, a situation blamed in large measure on the requirement of celibacy and the ban on marriage, but he said nothing would justify chanaging this element of the tradition of the Latin church.

Rather, he urged bishops to make all efforts to encourage vocations "to form new generations of priests.