One day in 1977, the morning mail brought a letter from Washington to the Grove City College administration building where, amidst leaded windows and other architectural mimickry of centuries past, Presbyterian minister Charles Sherrod MacKenzie presides.

Inside the envelope was a form that would pledge the signer's compliance with present and future rules stemming from a 1972 federal law banning sex discrimination in schools. Instead of signing it, MacKenzie consulted with his trustees and decided to challenge the government's right to require such a pledge.

One victory and two defeats later, MacKenzie's legal crusade has reached the Supreme Court. Grove City, a coed college with no record of sex discrimination, is identified with an attack on an important element of a key sex-discrimination law. The case received new attention recently when the Reagan administration filed a brief that meets the college halfway.

Depending on how the justices parse the issues, Grove City College's case could result in a sweeping affirmation--or a marked narrowing--of the scope of several major civil rights laws. The enforcement tool it attacks--a mandatory cutoff of federal funds to institutions that discriminate--is also a linchpin of laws barring racial discrimination and discrimination against disabled persons.

The main question: does Grove City College, which has long boasted that it takes nothing from the government, receive federal aid in the form of Pell grants? Several of its 2,200 students receive these grants, which the college contends represent aid to students only. The government contends that they are institutional aid subject to revocation if the compliance form is not signed.

However, in its recent brief to the Supreme Court, the government modified its earlier stand, saying that the grants are program-specific, and require only the office that receives them--not the whole school--to abide by the anti-discrimination provisions in Title IX of the Education Amendments of 1972.

To MacKenzie, even this modified approach stretches the law far beyond Congress' intent. Worse, he says, it uses regulations to do so. When he talks of the case, MacKenzie enunciates the word "regulations" with hostile precision, letting each syllable out slowly around his pipe.

"Regulations," he said in a recent interview, "assume you're guilty before you've done anything. There's a difference between having laws . . . and regulations. Grove City College has practiced non-discrimination for over 100 years . . . . Wherever we can cooperate with the government we want to. But there's a difference between cooperating and surrendering jurisdiction.

" . . . We feel we have a particular place as a school that integrates Judeo-Christian traditions into our programs. If we are ever forced to become an extension of the government we will be forced to abandon our values in education. Once we accede to the principle that we have to sign that form, that the government has jurisdiction, that's the camel's nose in the tent."

"This is the issue on which the school has chosen to make its stand," said Bruce Thielemann, dean of the chapel, "but the philosophy behind it has been here for decades."

Founded in 1876 as a teacher-training school, the institution has been dominated by a few sharply conservative families--including the family of J. Howard Pew, former president and board chairman of Sun Oil Co., and the Ketlers, Isaac the founder and Weir the son and college president for 40 years.

Chapel attendance is still compulsory for students. The president controls selection of speakers, chapel programs and special events. There was an uproar two years ago over a theatrical performance of "Of Mice and Men," when local residents complained about the use of the expletive, "Jesus Christ." An attempt to show the movie "The Rocky Horror Picture Show" was blocked.

There are 22 women on a faculty of 125, and three of 19 department heads are women--ratios that are consistent with national averages, according to Lesley Francis of the American Association of University Professors. Faculty are hired by administrators; there is no tenure.

The school's commitment to austerity extends to its finances, and has paid dividends. Tuition, room and board total about $4,600 this year--less than half the charges for most other private colleges. It has no debts, even for the $3 million computer science building under construction. Private contributions approaching $400,000 have paid for the lawsuit.

Educated by professors with an avid belief in free enterprise, many of the students go into business.

"We're training the middle managers," said one faculty member, although another pointed out that one graduate has headed the National Association of Manufacturers, another is an executive vice president of Alcoa and a third is an executive of Walt Disney Productions.

The most popular major is business administration, though the school still turns out teachers, a few music majors, and about a dozen commissioned Air Force officers who annually complete the Air Force Reserve Officers Training Corps program on campus.

MacKenzie sees no contradiction between his stand on the compliance form and the longstanding presence of ROTC on campus. "We can veto course content or the professors" the Air Force chooses, he says. "Besides, we've always been a flag-waving kind of place."

So has the town of 8,800 surrounding it. The philosophical distance between school and town is as narrow as the street separating the staid black sign and willow trees at the college entryway from the blue-and-white pennants of McNutt Motors.

Both town and college are resolutely Republican. Both ban liquor. Both spent most of the century in the comfortable economic shadow of nearby farms, slightly more distant coal mines and steel mills 60 miles south in Pittsburgh or 20 miles northwest in Youngstown, Ohio.

Now many fields have been abandoned to scrub trees, mills and mines have closed and Mercer County faces an economic blight. But Republicanism still flourishes.

"The campus is very homogeneous," Dean Thielemann said. "The kids look very much alike, think very much alike, come from similar homes." The student body is composed of men and women in equal proportions. Most students come from Pennsylvania; about a dozen are black.

The college's sense of its place is encapsulated in the chapel's procession of stained-glass windows, one side picturing biblical figures and the other enshrining John Calvin, George Washington, Abraham Lincoln and the first three college trustees.

Its sense of mission is partly expressed by one of its four required "Keystone" courses, the one called "The Social Dimension of Life." "Basically the course is about government regulations," said Lee Wishing, a 1983 graduate. "They spend a lot of time saying regulations aren't any good."