THERE ARE 29 religious sects among the Amish people, but all have one thing in common - they have tried to separate themselves from what they see as the vanities and worldiness of modern man.

Ironically, this attempt to set themselves apart has made them a curiosity that has attracted hordes of tourists. These, in turn, have attracted the construction of two concentrated strips of modern, crass commercialism in south-eastern Pennsylvania, where nearly half the nation's 20,000 Amish live.

One would be hard put to find a more antithetical situation. But it seems to work. You will see the "Pennsylvania Dutch" in buggies along Routes 340 and 30 east of Lancaster, Pa., threading their way among the traffic weaving in and out of the amusements, shops, motels, fast food restaurants and plain old rip-off joints along the two stretches. Get off those roads and you will have to drive carefully because you will frequently encounter relatively slow-traveling horse and buggies, sometimes meeting them head-on on an ancient covered bridge.

(The term "Dutch," used to refer to Amish, Mennonites, Moravians and other early German settlers in the area, is a misnomer. Other early area residents thought the new settlers were calling themselves "Dutch," when they actually were saying "Deutsch," which means "German.")

Some of the most beautiful pastoral country a weekend traveler from Washington can find lies 2 1/2 hours away along the back roads in Lancaster County, the home of these "plain people." We recommend the traveler make a first stop at the visitor's center at the juncture of U.S. 30 and Hempstead Road, northwest of Lancaster, for guidance and information. Probably the second stop should be planned for a quick lesson on what the Amish (Ahm'ish) are all about.

Three places do that - Amish Farm and House, Amish Village and Amish Homestead - all on a purely commercial basis, to be sure. We chose the Amish Homestead on Route 462, three miles east of Lancaster.

The commercialism hits like a sledgehammer. Upon paying the entry fee you are handed a packet of pamphlets pushing all sorts of come-ons, from a wax museum to hamburgers. The $2 a-head for a half-hour tour of the farm is a bit steep, but we wanted to learn more about the Amish and did, right down to the meaning of their clothing. After the guided tour, you can know which Amish persons you see are mourning, which are married, which have been baptized and whether they are traveling to a formal event.

The Amish Homestead tour is through part of a 71-acre farm run by an Amish family who are tenant farmers and live in one end of a 200-year-old house furnished as a typical Amish home. The part of the house the family lives in is off limits, but the tour includes the garden the housewife keeps for her table and the outbuildings, including the one where they store their buggies.

We learned that the family is Old House Amish, which means they pray in their own home or in the home of another such family on alternating Sundays because they feel churches represent vanity.

They apparently are not all that strict about shunning some modern ways, just as long as they do things a little differently from the rest of us. For example, they can use a propane stove for cooking, but if the average American has a white one, they paint theirs black. State law has forced them to use engines for some farm work, such as on milking machines. And electricity is required in the section of the house that is open to the public.

Because they are not open on Sundays, there are two other visits we made on Saturday.

One was to a market in Lancaster. There are three. The more historic and popular Central Market, hidden just off Penn Square, is open only on Tuesdays and Fridays, but the Southern Market, a few blocks away on Queen Street, is open until 3 p.m. on Saturdays. West End Market is open Tuesdays and Fridays and until noon Saturdays.

At the markets you can see all sorts of people, including Amish and their more liberal brethren, the Minnonites, selling farm produce. A self-guided walking tour of the city, which includes the markets and several other stops of interest, is available from the information center of West King Street.

One stop on North Prince Street, is the Fulton Opera House, appearing as it did in the 1850s, right down to the wooden statue of Robert Fulton of steamboat fame in a niche above the doorway. Tours of the opera house are available at a price, but as long as the box office is open you can see all you need to from the rear rows of the theater.

We then visited the monastic cloister at Ephrata (Eff-ruh-tuh) 10 miles northeast of Lancaster on Route 222. The cloister is open from 9-5 Saturdays and noon-5 Sundays at $1 a head for self-guided tours. It was founded in 1732 by Conrad Beissel, who believed in austerity and somplicity with the sole prupose of serving God. He, too, had fled from Germany.

There are few places in this country where wooden buildings as expansive as this, built before the Revolutionary War, are still standing. The site is operated now by the Pennslyvania Historical and Museum Commission, since the last residents of the cloister died in 1934. Watch your head when walking about in the buildings, as the doorways were built low to instill humility. Members of the cloister slept on benches in tiny cells, with wooden blocks as pillows.

The Vorspiel, a musical drama depicting life in the cloister, is performed Saturdays through Labor Day weekend and Sundays in August, at 9 p.m. for $3.50 per person.

We drove next to Litiz (Leh-teetz) about eight miles due north of Lancaster, where we had reserved a room at the General Sutter Inn. Lititz is a different part of "Pennsylvania Dutch" country. Not many Amish reside in that area, but it is just as rich in historic surroundings, primarily the legacy of Moravians.

The inn sits at the beginning of Main Street, just beyond a fountain in the middle of the intersection. Main Street's first four blocks are worth slow strolls to see house after house from the 18th century, houses as old - but seemingly less modified or modernized - as any houses in Georgetown or Alexandria.

You can learn about them from the menu if you eat at the inn. The flip side of the menu traces the history of the inn back to 1873, when Gen. John A. Sutter moved to Lititz for the curative waters of Lititz Springs, having suffered the rigors of the California Gold Rush, Sutter owned a million acres of land in Sacramento Valley when gold was discovered there in 1848. He was nearly trampled in the ensuing stampede for instant riches. Sutter's last years were spent on the road between Litiz and Washington, where he sought recompense from Congress - unsuccessfully - for his lost California land and for his military service in the Mexican campaigns. He died on one of the trips in 1890.

Aside from the history (the building actually dates from 1764 as the Zum Anker Inn), the meal is worth going for. The Goldwasser Room offers a varied menu, plus unlisted specials, with entrees priced from $5 to $10. We chose a couple of the specials: filet mignon stuffed with shellfish, at $10, and halibut with scallops at $8. Half bottles of domestic wine went for a restaurant-reasonable price of $2.25 and the entree price include soup, a generous salad bar, potatoes and crackers and cheese spread. We finished with coffee and Goldwasser, an orange liqueur with little gold flakes in it.

The menu explains that in Sutter's day, "the elite sprinkled gold dust in their alcoholic beverages supposedly for two reasons: one - to show their affluence; second - to relieve their arthritis."

Our bill, which included two cocktails, was a reasonable $28 plus tip. We needed a walk after the generous meal, so we walked down Main Street, looking at houses described on the menu.

The inn is not as good for sleeping as it is eating. There was a party the night we were there and a loud band played directly beneath the second floor sleeping quarters. Sleep was impossible until the band quit playing at 1 a.m. Otherwise, the room was comfortable enough, with cheerful flowered curtains and a television perched atop a yellow-painted chest of drawers.

Sunday we meandered through the lovely, rolling countryside, driving through towns with quaint names like Fertility, Bird-in-Hand. Intercourse, White Horse, Christiana and Paradise. If you don't care to drive the area, the Strasburg Railroad is an alternative. It provides a one-hour round-trip from Strasburg to near Paradise in picturesque, old-time railroad cars pulled by a steam engine, passing by Amish dwelling and farms and much of the country clear of electric wires.

Totally unconnected with Amish country is another attraction we visited on Sunday, the home of President James Buchanan in suburban Lancaster. His home, called Wheatland, is a 19th-century mansion restored to its original state with many of his own furnishings. It is open 10-5 daily and costs $2 each for a one-hour guided tour.

Among upcoming events in the area are the Kutztown Folk Festival July 2-9 between Reading and Allentown. Pennsylvania Dutch Days will be held July 26-31 at Hershey, where the chocolate company has its own theme park. Hersheypark, about 10 miles east of Harrisburg.

Some information sources include the Tourist Information Center at 1800 Hempstead Rd., Lancaster, Pa., 17601, or telephone (717) 393-9705 and the Mennonite Information Center. 2209 Mill Stream Rd., just off U.S. 30 closed Sundays, phone (717) 299-0954. Ephrata Cloister is 632 W. Main St., Ephrata, 17522, or (717) 733-6600. Wheatland is 1120 Marietta Ave., Lancaster, 17600, or (717) 392-8721. The General Sutter Inn address is 14 E. Main St., Lititz, 175-43, or (717) 626-2115.