Christopher Isherwood's life has taken him from the upper-middle-class world of England to the demimonde of pre-Hitler Berlin, from war in China to a Quaker refugee hostel in the U.S. from Hollywood screenwriting to Hindu philosophy. In the '30s he was regarded as the most promising British novelist of his generation, but when he emigrated in 1939 to the United States, his reputation gradually dec-declined. Now, in the last several years, as his artistic reputation has been revived, he has turned from novels to autobiography to come to terms with his past. In the process he has become an outspoken advocate of gay liberation.

"I think that kind of political action is one of the functions old age," says Isherwood, who turns 74 this month. Slight of stature, but looking hale and trim, Isherwood still speaks with a distinctly upper-class British accent despite having lived almost half his life in Los Angeles.

"As you become old, you automatically become, in sort of curious way, respectful, he says, sitting in the living room of his Santa Monica home overlooking a wooded canyon and the Pacific. People think. 'After all, with his white hair, he's past it at his age, so therefore it's rather impressive when he comes out and makes a fuss about it.' So in a word, I feel it's the duty of people like me to get in the act. It seems sort of contemptible for old people not to speak up. When the hell are you going to speak out?"

Isherwood's openness about his homosexuality is part of the re-examination of his life in his writings. "I write because I'm trying to study my life in retrospect, to find out what it is made of, what it is all about," he's said. In his early writings he interpreted his experience fictionally, but now he is returning to the same experience to treat them autobiographically.

"Insofer as it is fictional," he says, "It's rather like fishermen's stories, you know, you lie a bit. The fish get bigger or the circumstances get more difficult. In the absolutely autobiographical autobiographies I'm writing now, I don't allow myself to do that. I say precisely what happened. The difference is very considerable and well worth writing about all over again."

In "Christopher and His Kind" (1976), the memoir of his life from the age of 24 to 34, from his first visit to pre-war Berlin to his arrival in New York City with poet W. H. Auden, Isherwood candidly describes the homosexual relationships about which he remained so reticent in his novels. "At that time I didn't want to come out because it would have upset members of my family and would have been embarrassing for me, and more important, embarrassed the various people I was living with at any particular time.

"But I think much more deeply than that, really. To be quite honest, at that time I was trying to be another kind of writer, a writer who effaces himself, who says, 'Don't mind me, I'm going to tell you about him and her and them.'" 'Goodbye to Berlin'

In "Goodbye to Berlin" (1939), one of the two novels he wrote about the four years he spent teaching English statement about his artistic intentions: in Berlin, Isherwood made his famous "I am a camera with its shutter open, quite passive, recording, not thinking."

The characters Isherwood's camera eye recorded in "Goodbye to Berlin" and "The Last of Mr. Norris" (1935) were later popularized on stage and screen, first in "I Am a Camera," then in the musical "Cabaret."

Each succeeding version was less and less faithful to the nightmarish world of Isherwood's original work, but the performances of Julie Harris, in the play and film of "I Am a Camera," and Liza Minnelli, in "Cabaret," introduced the misadventures of his character Sally Bowles to a great many people who had never heard Isherwood's name.

Although the worldwide success of the plays and movies have made Isherwood famous, he sees little resemblance in them to the grim and haunting Berlin he knew. "A film that really conveys the sort of Berlin I had glimpses of," he says, "is Ingmar Bergman's otherwise not very satisfactory picture, "The Serpent's Egg.' The moment the picture came on. I smelled Berlin. For all its absurdity in other ways, it really brought back the smell of that time."

Berlin Novels, reissued in one volume as "The Berlin Story," may be the author's best known work but he says Germany has definitely faded into the past. "Christopher and His Kind" was the last equeezing of the orange. I laid the last of it to rest and hope never to have to refer to Germany again. I'm as deeply imbedded in American life as I ever was in Berlin."

Both personally and artistically Isherwood regards his years in California as the most important of his life. His two short novels, "A Single Man" (1964), and "A Meeting By the River" (1967), he considers his finest fiction. He and Don Bachardy, the painter with whom he's lived for the past 25 years, recently turned "A Meeting by the River" into a play. They expect to open it this fall either in New York or London with two highly regarded British actors, Keith Baxter and Simon Ward.

Of all his novels, however, Isherwood prefers "A Single Man." 'A Single Man'

The short novel describes a day in the life of George, an aging. British, homosexual college professor living near the ocean in Los Angeles.

Here the narrator is no longer self-effacing. The hell described is not that of other people, but the one he is living himself. The bleakness and loneliness of George's life in "A Single Man" represent not just the plight of a middle-aged homosexual, but that of all outsiders in our society. "In taking up the cause of one minority, that of homosexuals against the dictatorship of heterosexuals, I have spoken out for all minorities," says Isherwood.

Although the author identifies more closely with George than with any other protagonist in his novels, the two are not the same person. "I feel if I were really George I would kill myself," he says. George is a person he might have become had he not discovered Vedanta.

Isherwood's interest in Vedanta, which simply means Hindu philosophy, came after he had emigrated to America. He arrived in New York with his oldest friend, W. H. Auden, with whom he had collaborated on three plays and "Journey to War," a book about their travels to China during the war of 1938. Although Auden found New York to his liking, Isherwood preferred California, where he immediately found work in Hollywood.

Although few of his screenplays have been produced ("I always get paid; that's my proud boast," he says), Isherwood admits to enjoying screen-writing. "I'm not one of those people who goes around saying thats 'Oh, I'm a whore,' I despise people who 'Oh, I'm only doing it to support my says with loathing. "They all say, 'Oh, I'm only doing it to support my children' or some such trash. Rather shoot the children. They have no business to have children if in order to support them they have to do something which they regard as the vilest."

Other distiguished English writers and emigres like Aldous Huxley and Gerald Heard had already settled in Los Angeles before Isherwood, and through them he met the Hindu monk, Swami Prabhavananda. Their long relationship gradually transformed his life.

"Being the kind of creature I am, I never understand anything in my life except through people," he says. "All my money is based on my own intuition. What else has one? 'A Meeting by the River'

"In time I simply decided I dug this guy. He was absolutely not crazy nor a crook - for the record I should say he wasn't n the least bt homosexual ether. After billions of words, I came to the conclusion that he's got something. If somebody's got somethin, by definition, you have it too."

His experiences working at an American Friends Service Committee refugee hostel during the war confirmed many of the teachings of the Swami. "The Quakers had much to teach with other words," he says. "I learned a great deal of the same thing all over again, which is very reassuring, because you realize it's absolute truth," he laughs heartily. "I met saints everywhere, all over the place."

As he has throughout most of his life, Isherwood kept voluminous diaries of this period. He is returning to them now as the main source for the current volume of his autobiography. The book, which he expects to complete later this year, describes his experiences in Hollywood, his meeting with Swami Prabhavananda, and his relationship with him until his death in 1976.

He has written previously about Vedanta in several books of non-fiction and in his last novel. "A Meetin By the River." The river is the Ganges and the meeting a reunion of two brothers, one of whom is about to take his final vows as a Hindu monk. Isherwood's return to the subject of religious enlightenment, like his return to Berlin in "Christopher and His Kind," is to examine his own experiences as honesty as possible.

In all Isherwood's work of the last decade, from "A Meeting By the River" through his autobiographical works, there is a strong feeling of reconciliation, of coming to terms with his past. This is especially true in his first autobiographical work. "Kathleen and Frank" (1971), a memoir of his parents based on their letters to each other and on his mother's diary. 'Kathleen and Frank'

Frank, a lieutenant colonel in the British army, was killed fighting in France in 1915, when Isherwood was only 10. The young Isherwood rebelled against the image of a Hero-Father and all that he seemed to represent: patriotism, the public school system, a rigid sexual morality and class structure. At the same time he also rebelled against his strong-willed and domineering mother, deliberately flunking his exams at Cambridge so he could not be the don she wanted.

His departure for America in 1939, and his subsequent American citizenship in 1946, separated him from both "Mother and Motherland at one stroke." But in writing about his parents he rediscovered them as people instead of smybols, recognized his kinship with them, and his heredity. In the end he realized he was "far more closely interwoven with Kathleen and Frank than I had supposed, or liked to believe."

Isherwood attributes the reconcilitory spirit of his later works to many things, the perspective of age, his religious philosophy, the literary process itself. "There's no doubt that even the worst of us are much greater when we're engaged in literary activity," he says, "because you're trying to see all your characters, sub apecie aeternitatis . Under the great light of art, we demand charity, love and interest even when we deplore what the people are doing. If we were writing about Anita Bryant and John Briggs (the sponsor of an anti-gay initiative in California), we'd forgive them in five seconds because we'd see what brought them to this. They'd be instantly pardoned. And if people can be pardoned by novelists, then certainly they can be pardoned by God."

He pauses a moment, considering his life form the vantage point of his age. "you know, I'm in a very existential terminal situation now, about to turn 74. The real question that has to be asked somebody at my age is what about death?"

"People make a great mistake thinking you ought not to think about death. On the contrary, the more you think about it, the more sense of proportion you get. I always hesitate to say I feel quite all right about death, but I must say this is probably the happiest period of my life . . ." He stops, considers it again. "Yes, I really am very happy."