Gretchen Eick is part of a new political force that is shaping, if not determining, the House's crucial go-or-no decision this week on the MX intercontinental ballistic missile.

"We're going to win," Eick said, predicting without qualification that the House will refuse to provide the $988 million President Reagan is requesting to put the first MX missiles into production.

She is part of a growing number of religious groups which, after years of delivering lofty sermons on the evils of war, have decided to go down into the political lists to try to unhorse the experts on such complicated questions as whether deploying the 10-warhead MX would make nuclear war more or less likely.

Eick, Washington representative of the United Church of Christ, is allied with such religious groups as the American Baptist Churches, Episcopal Church, United Methodist Church and United Presbyterian Church in the battle of the MX.

They spent last weekend on the telephone urging their members in key Congressional districts to pressure their representatives to vote to delete MX procurement funds from the defense money bill. They contend that the MX is a first-strike weapon that will increase the possibility that either the United States or the Soviet Union, rather than risk having their missiles destroyed, will launch pre-emptive strikes.

The Reagan administration argues that deploying the MX will make nuclear war look more than ever like a losing proposition to the Kremlin, thus reducing the likelihood of one starting at all.

But members of Congress and their staff members trying to sell this argument to wavering colleagues before this week's crucial House voting conceded the religious groups have added an extra dimension to the lobbying.

"I can tell from our anti-MX mail that a lot of it has been generated by the churches," said a staff aide to a pro-MX congressman.

Why has it taken so long for church representatives to start playing political hardball, 37 years after the United States exploded the first atomic bomb and opened the nuclear age?

"It is related to a new sense of danger," replied Eick during an interview in her office in the Methodist Building on Capitol Hill where the phone never seems to stop ringing. "We're heading in such a frightening direction, and we've decided something can be done to change it. You have a president talking about nuclear war being possible.

"We're no longer being scared off because it is getting into a technical area. Even though we didn't have all the expertise, we won on the chemical weapons," for which Congress refused to authorize funds.

Besides mobilizing voters, distributing literature and buttonholing lawmakers all over the capitol, religious groups have been sending some of their most eloquent members on Capitol Hill to help build a case against such weapons the MX and Pershing nuclear missiles.

Leila Anderson, 84, a minister in the the United Churches of Christ, dramatized the power of this kind of non-expert lobbying against the MX before the House Appropriations subcommittee on defense last summer:

" . . . I came a long way by myself, driving almost 900 miles . . . . I am a birthright Republican and vote as I please. I have traveled in 31 countries . . . . I have lived through four wars . . . . For the first time in my life I see the possibility that we will be destroying our world and all the people in it . . . . "

Eick said it this kind of deep-seated fear among the citizenry that has fired up religious and other organizations, not any outside manipulation as Reagan has suggested in discussing the worldwide nuclear freeze movement. "We're not Soviet stooges," she said. "We are people who are against weapons which support the illusion that nuclear war can be fought and won."