Belgium finally celebrates the work of artist James Ensor

The monumental Belgian artist James Ensor (1860-1949), whose work is rarely exhibited in his native land, has come home. In what has been called the artistic high point of the summer and fall seasons here, nearly 300 paintings, drawings, and etchings, including his masterpiece, ''The Entry of Christ Into Brussels,'' have gone on display to large and enthusiastic crowds at the Koninklijk Museum (until Oct. 30).

To everyone, and particularly to his closest friends at the period's most advanced art school, Les Vingt (The Twenty), which he helped form, Ensor was ''the unknown.'' No biographer since has been able to penetrate the personal life and psyche of this modernist giant.

Born in the seaside resort town of Ostend, Ensor, who seemed to thrive on solitude, was rejected by society and by the art world of his time. He lived alone above his parents' curio shop, isolated culturally, socially, and professionally. There, he created macabre works that, according to New Yorker art critic Harold Rosenberg, vividly revealed his ''psychic estrangement'' - a ''nightmare theater in which the personae of his world are reflected as drunkards, masks, skeletons, demons, and corpses.''

But Ensor's pictures were ''saved'' from bathos and sensationalism, said Rosenberg, by ''the direct, felt quality of his handling, by his matter-of-factness and by their separation from the banal settings of 19 th-century genre painting.''

Ensor left Ostend infrequently (to attend art school for three years in Brussels and to visit art galleries in London several times), but unlike two contemporaries - Van Gogh and Nietzsche - he didn't seem to suffer from his ''psychic estrangement.''

The artist's alienation, in fact, only served to stimulate him to create even more daring works, and it was in his 20s and early 30s that he reached his creative peak.

Ensor, however, was ignored by certain critics, ridiculed by others, and the avant-garde of his day rejected him. (''The Entry of Christ Into Brussels,'' for example, was turned down for an exhibition arranged by Les Vingt.) Yet after his creative powers deserted him relatively early in life, he went on to live comfortably for more than half a century, reaping the ever-increasing rewards of his early talent. Through longevity and determination, Ensor, in fact, lived to see Belgium's King Albert name him a baron - probably the most gratifying event in his life, the perfect revenge on society and his critics.

Not unlike other artists of the period, Ensor took credit for influencing the development of virtually every artistic movement he could think of. But in reality, while he is admired today as an important contributor to various styles , uncertainties remain about his work and its impact on modern art. And like the man himself, his work doesn't fit in.

If there is anything that can be said categorically about Ensor, it is that he carried the 19th-century theme of the alienated and despised artist to the extreme. At 26, for example, he painted a self-portrait ''The Artist Decomposed, '' and at 28, he described in an etching ''my portrait in 1960'' - a pile of bones. ''My Portrait as a Skeleton'' followed.

With the exhibition of his work now on display in Antwerp, the artist has come home. And to judge from the large crowds turning out for it, they feel little alienation from him - even if he would still feel estranged from them.