IN Rebels of the Heavenly Kingdom, as in much of her previous work, Katherine Paterson writes about the difficult but enlightening processes through which young people who are prematurely left to their own resources become acquainted with the compromises and obligations that are necessary to survival in the adult world. It doesn't make a bit of difference whether the youth who undergoes this experience is a scared, lonely boy in 12th-century Japan (as in The Sign of the Chrysanthemum) or a troubled, troublesome girl in 20th-century America (as in The Great Gilly Hopkins); the constants are the sensitivity, humor and clarity with which Paterson considers the many nuances of her central theme.

Paterson treads a fine line between "juvenile" and "adult" fiction, and never has that line been finer than it is in Rebels of the Heavenly Kingdom. Indeed, she has written a more accomplished work of fiction, and certainly a deeper and more resonant one, than most of the novels written these days for an adult readership. Paterson obviously does write for a youthful audience-- readers between the ages of 12 and 16, approximately-- but she treats that audience as if it were grown-up: not in the manner of Judy Blume, the Jackie Susann of the acne-and-braces set, but in the manner of one mature person talking to another.

Rebels of the Heavenly Kingdom is set in mid-19th- century China, a time when the Manchu Empire was under intense internal pressure from rebellious Chinese nationalists. One insurgent group was the Taiping Tienkuo, "the Heavenly Kingdom of Great Peace," which had as its inspiration a rather peculiar blend of Oriental philosophy and Christianity learned from Western missionaries. An involuntary convert to its cause is 15-year- old Wang Lee, who had been abducted from his peasant home by "stupid and dishonorable rascals" and then rescued from these bandits by an 18-year-old girl named Mei Lin, a passionate devotee of the Heavenly Kingdom.

Gradually, as he lives among the membership of this "God-worshiping Society," Wang Lee finds himself attracted to its message: "In a world in which long-nose foreigners killed Chinese because they refused to poison their nation with opium, where Manchu armies killed Chinese peasants for a handful of rice, and where pirates and bandits killed anyone for no reason at all--in such a world it seemed quite wonderful to him that there were those who sang of human brotherhood and peace."

Yet such is the nature of man that even this peace- loving multitude becomes corrupted, and Wang Lee along with it. Its revolutionary forces, assembled to fight a holy war against the Manchu overlords, themselves become as bloodthirsty as any outlaw band. Wang Lee, though a mere boy, becomes an impassioned warrior of the Heavenly Kingdom who kills, wantonly and bloodily, out of an increasingly mad conviction that the cause of the kingdom is greater than any individual human life. It is a conviction in which he is abetted by Mei Lin, who herself becomes one of the most accomplished warriors in the army, a soldier of near-legendary achievements.

Wang Lee has learned to fend for himself, but in the process he has lost sight of the value and dignity of ordinary life, as expressed in the words of the Heavenly King of the Heavenly Kingdom of Great Peace: "You should not kill one innocent person or do one unrighteous act, even though it be to acquire an empire." Those words are the epigraph for Rebels of the Heavenly Kingdom, and they constitute its principal moral. But Katherine Paterson considers this moral without a trace of sermonizing or righteousness. The education to which she subjects Wang Lee--and, for that matter, Mei Lin-- is a hard one, in which nothing is learned without a measure of pain; the discovery that there are on earth no Heavenly Kingdoms does not come easily to him, and there is nothing facile about the manner in which Paterson leads him to it.

But then it is one of the many strengths of Paterson's fiction that, unlike so many who write for young readers, she always has her gaze set firmly on the realities of life. She makes Wang Lee's world actual in two ways: she gives us a wholly believable 19th-century China, and she gives us an experience that is entirely true to the way life works. In the sense that really matters, Rebels of the Heavenly Kingdom is a grown-up book--just as, at its conclusion, Wang Lee is himself a grown-up man.