The radiant smile on Helen Midboe's face bore witness to the depth of her feelings about yesterday's signing of the accord between the Lutheran and Roman Catholic churches.

"I'm very excited and very hopeful," Midboe said before she and two friends entered Washington National Cathedral to attend a Reformation Day service. "If the churches can work together in missions around the world, the effect would be much greater."

The idea of Lutherans and Catholics joining forces in faith and service was the theme repeated by several worshipers before yesterday's Reformation Day service, an annual Lutheran celebration of the day Martin Luther posted his 95 theses. The event took on greater significance with the signing in Germany of the Joint Declaration on the Doctrine of Justification by Lutheran and Roman Catholic church officials.

Nearly 500 years of disagreement--sometimes resulting in warfare--on whether man is saved by faith or deeds was brought to an end by the joint statement. Several people outside the Northwest Washington cathedral said that while the flocks of both churches got along fine, it was good to see the leaders of their denominations officially end the centuries-old dispute.

Lutherans Caryl Stalich, of Bethesda, and Florence Heacock, of Northwest Washington, said it made sense for the denominations to come together.

"The Catholic and Lutheran and Episcopal liturgies are so similar," Stalich said.

"I can go into a Catholic or Episcopal church and feel at home," agreed Heacock.

"This makes the leadership resolve the philosophical and political differences," Stalich added. "This is very historic and significant going into the millennium."

About 1,500 people attended the service, which was led by the Rev. Theodore F. Schneider, bishop of the Metropolitan Washington Synod of the Evangelical Lutheran Church in America; Bishop Ronald H. Haines, of the Episcopal Diocese of Washington; Auxiliary Bishop William E. Lori, of the Catholic Archdiocese of Washington; and Bishop Paul S. Loverde, of the Roman Catholic Diocese of Arlington.

The signing, Schneider said in an interview last week, symbolizes "the genuine interest in all of our churches to find fuller expressions of unity." It's "a scandal," he added, "that we tell husbands and wives to do things together, to attend church together," but that all members of the Christian family cannot find ways to share the same beliefs and worship experience.

Jean and Tim Unsworth, Chicago residents who attended the service, said many Catholics and Lutherans have long ignored the theological rift. "In Chicago, nearly half the marriages are mixed," said Tim Unsworth, a freelance writer for the National Catholic Reporter. "It's a nice pact, but it doesn't solve the power issues--birth control, remarriage, ordination of women"--practices opposed by the Catholic church.

The Rev. Paul Lee, ecumenical director for the Archdiocese of Washington, said both church communities have to be patient. "Reconciliation takes time. . . . Now we are able to say we are one in core faith. . . . All the other structural issues will follow."

The Rev. Richard Reichard, pastor at National Lutheran Home in Rockville, noting that much of the discussion has taken place at higher levels of the church, said the new spirit of cooperation will touch rank-and-file members.

"In my opinion, if God has his way, in time the denominations will come together," Reichard said. "Once we start working together and holding joint services, it will move out of the academic and into the practical realm of our daily worship experience."

Staff writer Bill Broadway contributed to this report.

CAPTION: Catholics and Lutherans worship together at Washington National Cathedral on Reformation Day.