Bill Milliken has shaved his beard, but he slides in and out of the Executive Office Building in Washington dressed as casually as he did when his hangout was an abandoned church building in the shadow of Atlanta Stadium.

Milliken's preoccupation is the rescue and education of slum kids. His chief weapon is a religious commitment so deep it borders on zealotry. His program goes by the unfamiliar name of Project Propinquity. But thanks to the patronage of First Lady Rosalynn Carter and other powerful friends, it is not likely to be unfamiliar for long.

At one level, the story of Project Propinquity is the touching personal tale of dedicated young people, "hustling" in the streets for help in rescuing a few of the victims of the urban environment from the lifetime of failure for which they clearly seemed destined.

At another level, it is the story of a largely invisible network of religious laymen, reaching into major foundations, businesses and now - due to the First Lady - into the White House itself.

Six years ago Milliken was "scrounging" for crumbs to keep a single store-front functioning in Atlanta. Now he is designing a prototype national program from an office next door to the White House and addressing officials of America's top corporations, invited to hear his ideas by the wife of the President and the director of the Office of Management and Budget.

A "federal connection" was almost made by hgih-level friends in the Nixon and Ford administrations; now it has been confirmed to the tune of $2.7 million for the coming year. And chances look promising for further expansion.

WHILE ACKNOWLEDGING the experimental nature of their efforts. Milliken and his colleagues have impressed many people who accept invitations to visit their projects. Last week, while Milliken was taking the President's son "Chip" Carter, through their South Bronx "street academy," his lieutenants were showing a visiting reporter some of the activities in their Atlanta headquarters - a remodeled church building in a rundown neighborhood.

Fourteen teenagers, most of them former dropouts and disciplinary problems, sat in a math class taught by Harold Finkelstein, a Ph.D. from Emory University.

The subject was the addition of positive and negative numbers - an invitation to instant boredom in the enervating heat. But the young teacher, swinging his gaze from the blackboard to the desks, rallied his students with rapid-fire questions. There was the same intense involvement you find in the last 30 seconds of a tight basketball game, everyone on the edge of his seat. And there were cheers, groans and laughter as they caught the excitement of the properties of additive inverses.

"You wouldn't believe that most of those kids were classified as functional illiterates by their schools, would you?" said a member of the Propinquity staff.

Across town, in the skyscraper office of the Trust Company of Georgia, which has contributed over $100,000 to the project, A.B. Padgett recalled his own reaction to such scenes: "I just couldn't grasp that you could use math as a motivational tool for dropouts, but they're doing it."

The basic idea of the program is one which long antedates Milliken and his associates - to concentrate the scattered social services around the one agency which "catches", at least temporarily, almost all the products of America's slums - the public school.

At one of the street academies here, there are 140 students and a staff of nine - five teachers, a family counselor, a student counselor and two street workers. To further the atmosphere of what center director Bobby Garrett calls "a very close family," each student has one of the staff members as a personal adviser. Each student has his adviser's home phone number and the adviser is available almost literally 24 hours a day for whatever problem may arise.

"We look for people who have lots of endurance and the ability to take stress," Garrett says. "Life doesn't stop at 5 o'clock, and if someone needs help at 2 a.m., we want to respond."

The local group is known symbolically as Exodus, Inc. Milliken and the Exodus directors, David Lewis and Neil Shorthouse, stress the need for small groups of students and for personal accountability by the professionals working with them.

"Kids fall between the cracks in big institutions and specialized services," Milliken says. "We insist that we operate on a scale where there is personal contact and individual accountability."

In practice, members of Garrett's staff say, that may mean that, if a student is chronically tardy, a staff member will swing by his house to urge him along; that, if an unmarried mother's grades drop because she is worried about her day-care arrangements for a child, a family counselor right in her school will learn of the problem quickly and go to work on it.

Milliken and his colleagues claim two other benefits for this approach. One is a boost in morale of the teachers who, Shorthouse says, "are blamed unfairly" for the failures of their pupils, "when the real problems are outside the school." The other is what they call the "synergistic effect" of concentrating school and social services at a single site, producing, they say, a greater cumulative effect than those same professionals could achieve acting individually on the same of students. Caring Plus Efficiency

HERE IN Atlanta, Exodus, Inc., operates two types of programs. It provides supplemental services for "bottom-rung" students in selected poverty-area elementary and high schools, trying to save them from flunking out or dropping out in frustration. And it operates two street academies for dropouts and youths with difficult disciplinary and learning problems, referred by school authorities or the courts and police.

Supported by eight separate state, county and local governmental units (some of which serve also as conduits for federal funds), six United Way agencies and private benefactions, Exodus, Inc., has slowly but steadily gained acceptance from the young people it serves and the professionals who work with it.

Milliken has been instrumental in launching a somewhat larger project in Indianapolis and has kept a smaller counterpart going in New York, where his own work began.

Dr. Helen Branch, who is evaluating the Atlanta project for the city's school system, says that, where Project Propinquity is working within conventional schools, student "attendance has improved - but not in a statistically significant way - and there are fewer course failures, indicating a little spark of improvement."

The teachers in the regular school program, she adds, are "very enthusiastic" about the improved attitude of the Propinquity students. And in the street academies, staffed entirely by Exodus, Inc., people, there has been "dramatic improvement in attendance and in reading skills."

Dr. Alonzo A. Crim, the Atlanta school superintendent, says. "This year we had 46 people graduate from this program - 46 who would not have made it otherwise. The great asset [of Milliken's group] is that they are caring people who have also shown they can operate withh great efficiency."

George Barry, the chief administrative officer of the city of Atlanta, says. "I deal with Federal programs for the city and most of them are money down a rathole. The difference with this program is the dedication of the people."

Business leaders like Padgett and management consultant Ted V. Fisher are equally enthusiastic about the cost-effectiveness of the projects and the workers' willingness, as Fisher says, "to live this life 24 hours a day. They really have a missionary zeal." The Individual's Importance

THAT DESCRIPTION is not entirely allegorical, for Milliken and many others in the group have their roots in a nondenominational Christian movement emphasizing individual commitments to community service.

Milliken was 20 years old and a college dropout in 1962 when he became involved in a Harlem storefront school project sponsored by a Christian youth group called Young Life. "We had only one gift," he says, "a feeling that any person was important enough to hang out with, to talk to on a bench, to love."

Young Life and its allied laymen's groups have provided a web of relationships for Milliken - a man whose conversation mixes street talk with preachments and professions of modesty with some name-dropping worthy of a Washington lobbyist.

After the fires of the 1960s civil rights and anti-war struggles, he found his way to Georgia, where he came under the influence of Clarence Jordan - Hamilton Jordan's uncle - the leader of an interracial religious commune surviving in the then-hostile atmosphere of Americus, Ga.

His faith reinforced by his contact with Jordan, he began working with a "Postal Academy" in Atlanta - one of a small network of street academies subsidized by the Postal Service when Winton M. (Red) Blount was its head in the Nixon Administration.

The crisis that brought Milliken together with the Carters was created when Blount left the Post Office to enter Alabama politics, and funds for his favorite project were suddenly canceled. Milliken knew Carter's faith-healer sister, Ruth Stapleton - "we'd spoken at some meetings together" - and she had given one of his books to Rosalynn Carter. Mrs. Stapleton brought Milliken to the mansion to meet then-Gov. Carter, and, according to Shorthouse, Carter gave them $5,000 from his emergency fund and signed a letter of endorsement that enabled them to raise another $100,000 from such other Atlanta bigwigs as Bert Lance, now OMB director, and Philip Alston and Anne Cox Chambers, both now ambassadors.

Milliken maintained his contact with Mrs. Carter, joined the Carters at the governor's mansion on the evening the Carter presidential candidacy was announced and made himself helpful in small ways.

But there were other contacts that looked even more promising at the time. Through officials of the Lilly Foundation in Indianapolis, a major supporter of Young Life, he met Richard J. Lugar, then mayor of Indianapolis and now a Republican senator from Indiana. Lugar, whose children attended Young Life camps, helped launch Project Propinquity in Indianapolis.

In January, 1976, Milliken shared honors at a Junior Chamber of Commerce banquet with Richard Cheney, then White House chief of staff, and infected Cheney with his own enthusiasm for the Propinquity concept.

There were several White House meetings, at one of which Lugar joined in urging that this was "a perfect urban program" for the Republicans to espouse. But in the end, Cheney says, "we were frustrated by the bureaucracy and we just weren't able to get it off the ground." The Carter Link

LAST Octobert, a few days before the second Carter-Ford debate, Milliken had a long meeting with Mrs. Carter at Atlanta airport, a meeting which, both of them say, deepened their friendship. Shorthouse says that Milliken gave Mrs. Carter "an important memo" on the Middle East from a Lilly Foundation official, "which Carter used in his foreign-policy debate." Milliken recalls that he asked Mrs. Carter to be sure her husband "doesn't forget about the poor."

In any event, Milliken found himself in Plains one day shortly before Christmas, conferring at length with Carter on the needs of the schools an city youth, while Andrew Young waited to discuss his United Nations appointment.

Shortly after the Inauguration, Milliken saw Secretary of Health, Education and Welfare Joseph A. Califano Jr., at the Carters' urging, then went back to the White House for lunch and dinner and further discussion of his plans.

"Jimmy told him to work up a plan." Mrs. Carter recalled in an interview recently, "and he sent me memos almost every week on it."

Milliken was given an office in the Executive Office Building, where he how spends three days a week, and Wib Walling, a veteran of Blount's abortive Postal Academies, was brought in to coordinate federal participation in the Propinquity program.

With Mrs. Carter's prodding, the bureaucratic road-blocks which had stymied Ford administration cooperation have been overcome.

"I have asked the department heads to see him," she said, "because I think what he's doing is so important. We spend money, money, money on these problems, and so many of the services don't even reach the people who need them. I also think it's important to get the private sector involved, and Bill does. I think what he does is just great, and he's so unselfish."

Last week, the interdepartmental committee created to coordinate agency liaison with Milliken's project assembled $2.7 million in federal funds - which will be roughly doubled by state and local and private sources - to finance an expansion of Propinquity projects in Atlanta. New York and Indianapolis.

In May, Lance, Lugar and Mrs. Carter sponsored and spoke at a breakfast for Milliken at which 230 businessmen heard him outline his program and solicit their support.

Within the next year, Milliken hopes to find local sponsorship for Propinquity projects in seven more cities - Hartford, Minneapolis-St. Paul, Pittsburgh, Los Angeles, Houston, Fayetteville, N.C., and Washington.

The combination of faith, fund-raising and the First Lady is putting Project Propinquity on the move.