Ambushed by Rothko

'Can you see it?" says the man in the Hawaiian shirt, pointing up at the purple canvas towering over us. "I've never been here before," he says, his shirt standing out wildly in the cool grey of the octagonal concrete room. "But I saw it in a matter of minutes. Can you see the figure of Jesus Christ our Lord on the Cross?"

I look politely into the misty bloom of the gigantic abstract work. It contains no images whatsoever, Christian or otherwise. I mumble something noncommittal, and he goes around pointing out Christ to everyone else in the room. They soon leave. I walk around staring at one colossal rectangle of sombre colour after another. A student comes in and kneels before a vast triptych that people choose to see as an altarpiece.

This is the Rothko Chapel in Houston , Texas . Art surrounds you here. Paintings on a majestic scale dominate each of its eight walls. There is little to interrupt their power, just the bare plaster, a few benches, and a couple of cushions on the floor. There are doorways, but they don't lead anywhere, except into a tiny alcove containing nothing. Their presence simply adds to the eeriness of this place, illuminated only by a skylight that softens the fierce afternoon sun. I am here on a pilgrimage to the greatest marriage of art and architecture in the US . But is this journey about art - or religion? The Rothko Chapel was designed to house the paintings of the abstract expressionist Mark Rothko, but it is also a sacred space, a non-denominational place of worship.

The chapel houses one of the two greatest cycles of Rothko paintings. The other is at the Tate Modern in London . On February 25 1970, the Tate took delivery of an astonishing gift: eight mural-sized paintings created by Rothko for the Four Seasons restaurant in New York 's Seagram Building , but judged by him to be totally inappropriate for the superficial, noisy, distracting setting of an expensive restaurant. Motivated by complicated reasons of his own - which included his pursuit of a tax break, his desire to insult New York's Museum of Modern Art (MoMA), and his admiration for the Tate's Turners - Rothko gave his most accessible, moving and enduring paintings to Britain. Long before I ever saw a Jackson Pollock, I would sit in the Rothko Room at the Tate, not depressed as some people say they are by this man's art, but awed and exhilarated. The day the Tate took delivery of the paintings, they also received some horrible news: Rothko had killed himself in his Manhattan studio, slicing open arteries in both arms and bleeding to death.

Rothko had painted the Seagram murals at the end of the 1950s. They herald a new darkness - literally - in his paintings, which had previously exhibited such brightness and vibrancy. While still marvels of colour, they feature deep shades, reds, purples, blacks, with frame-like forms painted over bloody depths, as if the canvases were windows on to the birth or death of the cosmos. I can think of few paintings that absorb me more, but I have always longed to see the place where Rothko took his pursuit of what he called the "tragic" to its ultimate extreme. Rothko believed that all serious art was about death, and the chapel was his last word, his crowning achievement.

In 1964, Texas art collectors and oil millionaires Dominique and John de Menil commissioned Rothko to create a cycle of abstract paintings for a chapel in Houston . Rothko built a full-size mock-up of the space in his Manhattan studio and painted 14 canvases to be set in various groupings around its walls. He used a pulley to adjust their height, allowing him to decide their exact locations. The only thing needed now was the building. But Rothko didn't wait around. He killed himself before the chapel was finished. It finally opened, with everything done as he wished, a year after his death.

With an exhibition dedicated to Rothko's final years about to open at Tate Modern, this is an ideal time to visit one of America 's greatest and strangest monuments: a chapel created by a modern artist who had no religious beliefs. Tate Modern's exhibition will bring together its Seagram murals with the other paintings he created for the restaurant, and show how his palette continued to darken right up to the end of his life. Any chance to contemplate Rothko's work should be grabbed at. He is one of the greatest abstract painters of the 20th century, one of the supreme US artists. In Edgar Allan Poe's story The Fall of the House of Usher, the doomed, hypersensitive Roderick Usher plays blues-like guitar music and paints abstractions that suck the mind into their desolation: it is an uncanny prophesy of the noble despair of Rothko, who is of all America's creative giants the most pessimistic.

I stopped at New York on my way to Texas . Rothko himself didn't get much further. He never visited Houston , and probably wouldn't have appreciated it much, judging from his letters to friends. "This physical and human desert," he called Boulder , Colorado . It made him long for "that island of Paradise which is NY City". Rothko came to his island of paradise in 1923, when he was 20. He had just dropped out of Yale University , where he stuck out a mile as an impoverished Russian Jewish immigrant. He headed for Manhattan and struggled to become an artist. His path was slow and painful but, by the 1930s, he was part of the Greenwich Village avant-garde that was about to give birth to abstract expressionism. In his touching 1936 self-portrait, his vulnerability, with eyes shielded neurotically behind thick blue glasses, is striking. In abstract art, he found a way to express the pain and longing those frames hid.

Rothko and his peers all had political and social consciences, fired up by the 1930s Depression and the Roosevelt administration's willingness to pay artists to paint murals for public buildings. Many 1930s and 1940s murals still grace Manhattan buildings today. This mural fervour shaped Rothko and is fundamental to understanding his passion for big pictures, his desire to "make a place" with art. This dream, tried and aborted in the Seagram works due to his disdain for the restaurant setting, finally came to fruition in the chapel.

Works at MoMA show how Rothko's generation, who were escaping from social realism into the abstract and the mythic, all thought big - and not just physically. There's a freedom of movement in their paintings, a feeling of space for the eye and the mind to roam. Jackson Pollock's One is a forest your imagination strolls into. Barnett Newman's Vir Heroicus Sublimis is even vaster. And then you come to the Rothkos. Pollock's epic canvases are horizontal, like cinema screens. Rothko's - such as Number 10, 1950, which once belonged to the architect Philip Johnson - are vertical, like skyscrapers. Rectangles of colour are layered over one another like windows in a tall building. These are paintings that belong in apartments high above the city streets. They speak of power, grandeur - and of wealth.

Of all these radicals, Rothko ultimately became the most seductive to art collectors. This is because, despite his slow learning, he turned out to be one of the most talented colourists of his century. In MoMA, you can compare his paintings directly with Matisse's colour field masterpiece The Red Studio - and Rothko stands up to it. His 1950s paintings are collisions and dances of colour. They are bright like Van Gogh's Sunflowers, with a lonely soul's anxious lust for life. They communicate isolation and unease in their closed rectangles, even as they delight the eye with clouds of colour. Edges are misty, ambiguous. Colour drowns you and absorbs you. And yet it refuses to give you the satisfaction of a fixed meaning, a story, let alone a picture.

Rothko's paintings sold well. They are, after all, so beautiful. But he was unhappy in his success, uneasy in his skin. By the 1960s, he was rich enough to live on

Rothko did not paint his cycle of dark paintings for the Menils' chapel because he was a religious man. He leapt at the chance to create a chapel because it was the type of cultural space that came closest to his ideal of making a "place" with art. Tell that to Houstonians. To spend a couple of days at the Rothko Chapel is to be at once impressed and silently troubled. Locals use this place. In fact, they love it. They come not just as tourists but to meditate, pray, and talk sombrely. They see it as a religious place and the art as spiritual. It is called a chapel, after all, and most Americans believe in God.

On a Sunday morning I meet a religious group who have just had a meeting inside. They take their inspiration, they explain, from the apocryphal Gospel of Mary. Their leader, Betty Adam, explains Rothko's art to me as her group nod: "These paintings are about Good Friday. This is Mark Rothko's dark period. But as you face the front of the chapel, the painting becomes more mauve and there's more light."

The group are "seekers". Their meeting in the Rothko Chapel seems little more than a rambling spiritual chat, almost free association. One man says he's finally come to understand Bob Dylan's lyric, "failure's no success at all". I'm tempted to tell him he's missing the point, that the line's meaningless without its context, its preceding line. It seems to me these people, and the other sincere believers I meet here, are missing the point about Rothko, too.

This chapel has been here nearly 40 years, yet it has never really become a stop for art tourists. It's a living communal entity: coming here is not so different from visiting Baroque churches in Italy , and seeing the rituals that go on within them. It is not, as some critics claim, an austere, dead, modernist monument. It's a living chapel. People sing and play music here. But maybe they should look around a bit more - because this is one of the most compelling rooms I have ever been in. Its art simply swallows you up.

The room is an octagon, which has a fascinating visual effect. As you walk in from the small lobby and see paintings ahead, there is the feeling of ambush: Rothko has got you surrounded. It's impossible to get away from his overarching vision. His paintings are bigger - much bigger - than you are. They are juxtaposed with those doorways that lead nowhere, that powerfully evoke the sinister closed doors at the corners of Michelangelo's New Sacristy at San Lorenzo in Florence . Michelangelo used sealed doors and sealed windows for one reason: to suggest death. Rothko's doorways are also portals of death. And they invite you to see the paintings as portals, too. For these are vertical rectangles, like giant black doors. They tower over you and pull you towards their madness. Stand close to one and you feel like you're about to totter into a void.

The interpretation - repeated to me and originated by Dominique de Menil - of the chapel as a progress towards the lighter, warmer colours of the "altarpiece" (an arrangement of three purple paintings that faces the entrance to the room) is inaccurate. The chapel does not have a single focal point. It does not offer progress towards a consoling vision. Instead, the last picture you see is the one you face as you walk towards the exit. It is the most oppressive of all. A black rectangle pierces a brownish container. It is a blackness of utter desolation, like looking into a waiting coffin. Any illusion of paradise the chapel might have engendered is dashed to pieces.

And Rothko planned it this way. His chapel is one of the most overwhelming syntheses of art and architecture in the world. It is as compelling as the great Italian religious interiors he admired, yet as terrifying as Munch's Scream. It is a tragic theatre of emptiness, death's antechamber, the self-expression of a suicide. As such, the Rothko Chapel was destined to be misunderstood. Had it been understood, it would not have been built.