Despite all its problems, San Francisco is still a great city for urban explorers. There’s something unexpected around every corner — a marvelous neighborhood restaurant, a quiet lake in McLaren Park, a smile from a pretty young woman on the J streetcar.

Or finding a church that is an architectural treasure hidden in plain sight for 125 years on a corner in Pacific Heights. This is the Swedenborgian church at the corner of Washington and Lyon streets, a building that is both famous — it’s a national historic landmark, like the cable cars — and inconspicuous.

The building does not draw attention to itself; it is not one of those churches that shouts to the heavens of the glory of God. It is built on a hillside with a stone wall on two sides; the church itself is brick with a red tile roof. The entrance is a simple arched gate that leads to a garden.

“You pass through nature to reach the sanctuary,” said John Gaul, a longtime member of the congregation.

That is one of the ideas behind the church and the religion it represents. This small Protestant denomination follows the teachings of Emanuel Swedenborg, an 18th century scientist and philosopher who believed in a new approach to the Christian faith. Among other things, Swedenborgians believe that the spiritual world is always present in everyday life. The teachings spread around the world, and reached San Francisco in 1852.

Even in the chaotic times that followed the California Gold Rush, the simpler teachings of the New Church, as it was called then, attracted many adherents. Among them was Joseph Worcester, a Swedenborgian minister, who was born on the East Coast and moved west in the 1860s. He had lived for a while in Yosemite Valley and had become a friend of John Muir and William Keith, one of California’s most noted landscape painters. Worcester, Gaul said, was interested in “high thought and simple living.”

The 1890s were a time of an intellectual renaissance in the Bay Area. San Francisco had more than 300,000 people and was the eighth largest city in the United States. The city attracted many artists, writers and thinkers — and architects, like A. Page Brown, Bernard Maybeck, Willis Polk, A.C. Schweinfurth and a young Julia Morgan.

Worcester was interested in building a kind of mini-cathedral of the Swedenborgian faith, “a sanctuary that would suggest the peaceful presence of God in all things,” the church says.

It was a cooperative effort: Brown, Maybeck, Schweinfurth and Worcester, and artists Keith and Bruce Porter all worked together.

Worcester, a talented architect himself, was “the magnet that attracted them all,” Gaul said. The result was what Gaul calls “a simple church unlike any other.”

It was built in the then-startlingly new Arts and Crafts style, which combined simplicity with a kind of urban elegance. The new Swedenborgian church was nothing like the carpenter’s Gothic Victorian houses on other blocks of Lyon Street.

It was not so much architecture, Brown said, as “the poetry of architecture.”

Brown was at the peak of his career when he worked on the Swedenborgian church. He had just finished designing the Ferry Building, San Francisco’s largest project at the time. Maybeck’s best work was still ahead of him — hundreds of homes and office buildings and his masterpiece, the 1915 Palace of Fine Arts.

The interior of the Swedenborgian church is the heart of the matter. The main sanctuary looks less like a church than a place for discussion and meditation. It is both simple and complex. The ceiling is supported by arches made of the trunks of madrone trees with the bark left on. There are two stained glass windows, and murals of the four seasons painted by Keith. There are no pews. Instead, there are 80 handmade maple-wood chairs, the seats made of tule from the Sacramento-San Joaquin River Delta.

There is even a fireplace in the back of the sanctuary, which gives the place a distinct feel.

“It’s a casual church, like a home,” said Andrew Dodd, a member of the church.

In some ways, the church with its quiet charm is part of another, older San Francisco, when modesty and simplicity were valued much more than they are today.

The building, said church member Nancy Leras, “offers serenity in the midst of the madness of the city.”

Regular services are held every Sunday morning at 11, and for the 125th anniversary, the church is offering a free series of talks and films by authors and artists. They will be held on two weekends, starting March 13. For information go to https://sfswedenborgian.org.

“If people come,” said Dodd, “they will discover a place they never knew existed.”

Carl Nolte is a San Francisco Chronicle staff writer. Email: cnolte@sfchronicle.com Twitter: @CarlnolteSF