The heathen hordes are still buried in their beds when Teresa Reeder arrives. It is 5:30 on a Sunday morning, yet Teresa and her teen-age cleanup crew are busy as bees. Up one row, down the next they go, picking up beer cans and stray popcorn. They are just finishing when the blue pickup truck zooms up.

Four men get out. For the next few minutes, they unload things, more things than it would seem the truck could hold: a portable organ, a lectern, chairs, a sound system. Then they spend a few more minutes arranging it all just so.

By now, 34 cars of all description have congregated. All simply park. Not a soul gets out. And not a flicker of disbelief is visible when organist Nan Burke plays a few bars, and C. Gordon Clews declares: "Father, we thank Thee for the freedom to assemble as we are this morning."

Yup, you guessed it, it's church. But there are no steeples or pews, few suits or dresses, no ushers, no hymnals, no stained glass or candles. For this is church in a drive-in movie theater.

In a what? There could hardly be a setting less pastoral, what with the giant, empty silver screen staring blankly down on everyone, and last night's unfinished orange drink moldering around the grounds.

"All I can say is, I've never believed much in ceremony or ritual," says the Rev. Gordon Clews, who devised the service on wheels. All one can reply is: evidently not.

Where "Bingo Long" and "Jaws 2" fill the ABC Drive-In in Oxon Hill with sound and fury each evening, Rev. Clews takes over on Sunday mornings, as he has for the last four summers. The hymmns, the solos and Clews' sermon are given from a makeshift platform atop the drive-in's concession booth. Parishioners come as they are, and listen to the service by hanging the theater's sound boxes on their car windows. All faiths are welcome.

In almost every respect, the service is the same as the one Clews gives 3 1/2 hours later at home base: the Providence United Methodist Church of Friendly, Md. The only difference is the way the offering is collected. When someone knocks on your car window and hands you a basket, it is much more difficult to say you gave at the office.

With a parish of only slightly more than 300, Clews noticed a severe attendance drop on summer Sundays when he first took over in Friendly four years ago.

The reason, he reasoned, was that his largely young congregation was choosing to take the kids to King's Dominion instead of the Lord's. The solution: Give them an early service, in an offbeat place, with free coffee and doughnuts afterward. The result: a church service that Clews says is "rated G for God" - and which parishioners say they wouldn't miss for all the tea in China.

"It's different; that's most of it," said Linda Patterson, 26, a nurse, who comes every week with her husband in their bronze Chevy van. "It's less formal, and I love anything less formal. If you go to church, you've got to put your dress on and all that. Religion's all in your head, anyway. You get the same thing out of it here as at church."

For Grace Mullinix, of Clinton, the 7:30 a.m. starting hour overrides any negative feeling she may have about the location.

"This is the prettiest time of the morning, even if this may not be the prettiest place to appreciate it," she said. "It's a little rough sometimes getting up for this service, but for some reason or other, the birds seem to chirp here."

When Clews first decided to fuse celuloid with salvation in 1975, he wasn't sure the idea would work. The amount of space and the location of the theater were perfect. But if Fred Wineland, the owner of the theater and Maryland's secretary of state, demanded a rental fee, Clews wasn't sure he could pay it.

Wineland did just the opposite.Not only did he charge nothing, but he built Clews' makeshift pulpit atop the concession stand for nothing. Wineland has not attended a Sunday service at his theater, but two of his employes are regulars: Herb Barger and his son Harold, both projectionists at the ABC at one time or another.

Although the ABC congregation is the only one on wheels in the Washington area, the idea has long been popular in Florida and California. And it is not the first time Clews has tried it with his own ministry. As a young minister in the 1950s in western Maryland, he did the same thing.

"I can't claim it's unique or original," he said. "It just seems to work."

It also seems to grow. Where 10 cars were regulars the first summer, 25 or 30 (containing about 75 people) can now be expected just about every summer Sunday. In addition, word of mouth has occasionally brought travelers from as far away as the Midwest to worship from behind their steering wheels.

Clews has even retooled a little of his leisure time as a result of his Sunday service. He confesses to having become a bit of a drive-in movie addict, "although most are a little lengthy for me."

Gordon Clews takes his ministry back indoors from September to June, because of the weather. But he wishes he didn't have to. "Out here is great," he said, as the four men were packing up the pickup. "You feel more prophetic talking under the sky like this."