Sorrowing Catholics, joined by Christians of other denominations, filled St. Mathew's Cathedral here yesterday for a mass of resurrection for Pope Paul VI, who died Sunday.

With virtually no publicity and little advance notice, nearly 1300 people gathered at the cathedral to hear the ancient words of consolation and assurance of eternal life for those who have kept the faith.

In the quiet, compassionate tones of a simple parish priest comforting a bereaved parishoner. William Cardinal Baum assured the congregation that Pope Paul VI was indeed one who had kept the faith and was thereby assured of the eternal fellowship of the saints.

"Today we offer to God the earthly life of Pope Paul VI," said Baum, who was dressed in the purple robes of mourning that all cardinals of the church will wear until the end of the nine-day mourning period for Pope Paul.

"We can say today he has kept the faith," the cardinal continued, as prelude to a litany like recounting of the ways in which the dead pope had been true to his calling as Christian leader.

"He has kept the faith by an unwavering adherence to Christ," Baum said. "He has kept the faith by his absolute fidelity to the Second Vatican Council . . . by insisting on the demands which the faith makes in our behavior in the world . . . by doing everything in his power to encourage a common proclamation of the (Christian) faith" and to erase the divisions among Christians "which were so painful to him."

Baum concluded his homily with a prayer to God in which he said. "You have taken Pope Paul VI from us but we are confident that You have have not abandoned us."

A handful of Protestant and Eastern Orthodox leaders who had made early morning calls to the pope's death were quickly invited to join in the processional, along with scores of diocesan clergy. Cardinal Baum acknowledged their presence as well as that of Mayor and Mrs. Walter E. Washington and D.C. Council President Sterlin Tucker with thanks.

But it was the people who crowded spontaneously into the ornate Romanesque church who bore the most impressive silent testimony to devotion to the Church. They were all races, all ages and several women carried tiny babies. There were almost as many men as women, some wearing the work clothes of the job they had slipped away from on their lunch hour.

Yesterday's service was arranged hastily, the cardinal explained, in order that he could preside over it before leaving for Rome this evening to attend the pope's funeral on Saturday and take part in the subquent conclave that will elect his successor.

Later this week, the apostolic delegate, to the United States, Archbishop Jean Jadot, will preside over another service at St. Matthew's that will be keyed to the rites in Rome.

In Arlington, Bishop Thomas J. Welsh will conduct a pontifical requiem mass at the Cathedral of St. Thomas More on Wednesday at 7:30. Heads of other Christian churches in the area have been invited to participate.

In Baltimore, Archbishop William Borders and Lawrence Cardinal Shehan will join in a mass of the resurrection at the Basilica of the Assumption at noon today before both churchmen fly to Rome for the papal funeral.

Special services will be held at St. Mary's Cathedral in Baltimore on Thursday, Friday and Saturday evenings and 7:30. The Saturday service will be an economical prayer service "for all peoples in mourning for Pope Paul," the archdiocese announced. Protestant leaders will join with Bishop R. Austin Murphy, vicar general of the archdiocese, in the Thursday service.

Bishop Walter F. Sullivan of Richmond will celebrate a memorial mass at 7:30 tonight at Ascension Parish in Virginia Beach and at 8 p.m. tomorrow at the Sacred Heart Cathedral in Richmond.