TUMBANG SAAN, INDONESIA — In this village near the heart of Borneo’s great, dissolving rainforest, Udatn is regarded as a man of deep spiritual knowledge.

Of all the people in this tiny settlement, he speaks better than any other the esoteric language of the Sangiyang, the spirits and ancestors of the upper world, known simply as “Above.” His is a key role in the rituals of Kaharingan, one of a number of names for the ancestor-worshipping religion of Borneo’s indigenous forest people, the Dayak.

“In the beginning, when God separated the darkness and the light, there was Kaharingan,” said Mr. Udatn, as he sat smoking a wooden pipe on the floor of his stilt home. (Like many Indonesians Mr. Udatn uses only one name.)

The Indonesian government thinks otherwise. The world’s most populous Muslim-majority country is no Islamic state, but it is a religious one. Every citizen must subscribe to one of six official creeds: Islam, Roman Catholicism, Protestantism, Buddhism, Confucianism or Hinduism. Kaharingan, like dozens of other native faiths, does not officially exist.