The nation's Roman Catholic bishops said this week that programs aimed at helping the poor should be shielded from the across-the-board cuts called for by the Gramm-Rudman-Hollings balanced budget law.

The bishops said they agreed with the "consensus that the deficit must be systematically reduced." But they said that principles of "distributive and social justice require that when the federal budget is cut, the most vulnerable people in our society should be protected from the effects of these cuts."

A statement issued by the administrative board of the U.S. Catholic Conference, the social action arm of the American hierarchy, said decisions on where to cut should involve "moral decisions about justice and fairness and moral judgments about how best to provide for the common defense and promote the general welfare."

They urged that cuts be "allocated fairly between defense and nondefense spending." They said that "arms control criteria as well as fiscal criteria" should be used to trim defense spending.

They proposed that "major weapons systems, which are of questionable effectiveness but which are certain to cost large sums of money, should not be pursued in this time of budget stringency." Saturday Services Set

Oak Chapel United Methodist Church in Silver Spring is trying something new -- for Methodists. The congregation has added a Saturday evening service to its weekly schedule.

The traditional 11 a.m. Sunday service, which will continue, "is an outgrowth of the days when farmers had to milk their cows and then clean up before coming to church," said the Rev. Jim Skillington, the pastor.

The 6:30 p.m. Saturday service, long a fixture in many Catholic parishes, has been added for those who find it difficult to attend on Sunday, he said. Headquarters in Dispute

A decision last month to choose Milwaukee as the national headquarters of the still-to-be-constituted Evangelical Lutheran Church in America shows signs of coming unstuck.

The choice of Milwaukee, a compromise between advocates of Chicago and of Minneapolis, has been widely attacked as not cosmopolitan enough.

The 70-member body guiding the merger of the American Lutheran Church, the Lutheran Church in America and the Association of Evangelical Lutheran Churches selected Milwaukee after an eight-hour debate.

But the decision has been so roundly criticized that it may be reconsidered, church leaders say, when the commission meets in June and at national meetings of the three merging bodies later in the summer.

The actual merger is scheduled for 1988.

Religion Bias Ruling A federal court in California has ruled that San Francisco International Airport discriminated against religion by trying to avoid renting space to a Christian Science Reading Room.

The reading room had been operating at the airport for 27 years. When the airport was renovated in 1984, municipal officials tried to evict the reading room on the grounds that renting space to a church violated church-state separation.

But the court ruled that "there is nothing to suggest that the airport endorsed the religious views and beliefs of the reading room or gave favored status to religion in general."

Renting space for a reading room constitutes no more an endorsement of a particular religion than does renting space for bars constitute endorsing alcohol, the court said. People

Rabbi Jeffrey A. Wohlberg, rabbi of Beth El Temple in Harrisburg, Pa., has been elected senior rabbi of Adas Israel Congregation. He will succeed Rabbi Stanley Rabinowitz, who is retiring in June.

Adas Israel also has named Rabbi Avis Dimond Miller as assistant rabbi, and is the first Conservative congregation in the area to have a woman rabbi. on its staff . . . .

Robert Billings, who was appointed by President Reagan during his first term as a senior officer of the Department of Education, is the new headmaster of Riverdale Baptist School . . . .

Beth Cohen, who is president of the Baltimore-Washington-Richmond Synagogue Administrators Association, will become executive director of Temple Sinai next week.