Three denominations -- Roman Catholic, United Methodist and Episcopal -- account for the religious affiliations of 51 percent of the members of the 98th Congress.

A list of congressional religious affiliations, prepared by Albert J. Menendez, director of research for Americans United for Separation of Church and State, shows that the affiliations of the other 49 percent are divided among 22 other denominations and the category "no affiliation."

Catholics, already in first place in the last Congress, gained six seats in the 98th Congress for a total of 141; Methodists increased by three seats for a total of 73, recapturing second place held by Episcopalians in 1980, and the loss of 10 seats put Episcoplians in third place with a total of 61.

Other denominations with high representation in Congress are Presbyterians, in fourth place with 56 members, and Baptists, still in fifth place despite a reduction of eight seats. The Jewish membership is up five to 38 to hold sixth place, while Lutherans, with a gain of two seats and total of 25, barely retained their small edge over nondenominational Protestants who increased from 19 to 24 members.

Menendez said he prepared the list after he became concerned that "nobody else was going to do it."

We are a denominational society. It still defines the way people think of themselves and the way they worship," Menendez said.

Pointing to the more than 50 religious lobbies in Washington, Menendez noted a growing trend for church-connected lobbying. "Denominations have taken stands on political issues," he said. "Lobbying offices here are generally denominational, so people still think in those terms."

The results of his study were contained in an article in the January issue of Church and State magazine, of which Menendez is associate editor. The general conclusion of his article, "The Changing Religious Profile of Congress," was that a member's religious affiliation means little in terms of how he or she votes.

But Menendez reports that religious affiliation still has some bearing on legislation of clearcut religious significance such as school prayer, abortions and tuition tax credits for private schools.

There is a trend toward religious pluralism, his article said. "Groups which were once out of the mainstream of American politics have now moved to a position of greater power. Catholics, Jews, Lutherans, Mormons and Eastern Orthodox Christians now have their highest representation in Congress.

"By contrast traditionally dominant "mainline" Portestant churches (Episcopal, Methodist, Presbyterian, United Church of Christ and Disciples of Christ) have declined steadily in congressional membership for over a decade."

Other religious-political highlights of the study:

* Jews, Unitarians, Roman Catholics and Baptists are molre likely to be Democrats than Republicans, but the long-range trend has favored Republican gains among Catholics, Baptists and Jews.

* There are almost 50 percent more Catholics in Congress today than there were in 1962, while the number of Jews has tripled since then.

* Catholic candidates are finding increasing acceptance all over the country, not just in traditionally Catholic states.

* All five Utah members are Mormon and Republican. (No other state has uniformity of religious affiliation).

* The relationship of ethnicity to religion is still firm but declining.

* Every member of the 97th Congress had a religious affiliation, but after the 1982 election the House of Representatives had five members with no religious affiliation.

"In America there is still a little bit of a stigma for somebody seeking public office not to have a religious affiliation," sand Menendez.

Yet the preference of Harold Washington (D-Ill.) for no affiliation apparently didn't raise many eyebrows. He was elected mayor of Chicago earlier this month.

The vacancies left by Washington's victory and the death of Phillip Burton (D-Calif.) have not been filled.