The Reihokan exists because Koyasan was losing its art. For centuries, the mountain's religious treasures were scattered across dozens of sub-temples, locked in storerooms or displayed in dimly lit halls where the act of liturgical use -- unrolling scrolls for visitors, exposing silk paintings to candle smoke -- slowly degraded them. Then came the Meiji Restoration. In the late nineteenth century, the new government enacted shinbutsu bunri, a forced separation of Shinto and Buddhism that stripped Buddhist temples of state support and left many destitute. Koyasan's monks, suddenly impoverished, began selling artworks to collectors in Tokyo, Kyoto, and Nara. Some pieces left the country entirely. The government responded with a series of cultural property laws beginning in 1897, but the damage was real. In 1921, supporters and Kongobu-ji Temple funded the construction of the Reihokan to consolidate and protect what remained.

The museum's most spectacular holdings are its sutra collections. A massive set of 4,296 handscrolls, known collectively as the Chusonji sutras, were originally commissioned by Fujiwara no Kiyohira between 1117 and 1126, dedicated to Chusonji temple in northern Japan, and later presented to Kongobu-ji by Toyotomi Hidetsugu. The scrolls are written in gold and silver ink on indigo blue paper and decorated with delicate paintings. Each one is a small masterpiece of calligraphy and illumination. Alongside these sit eighth-century Nara period sutras, including copies of the Konkōmyo Saishoo Sutra once enshrined in state-sponsored temples founded by Emperor Shomu for the protection of Japan. One of these copies is unusual for having 34 characters per line instead of the standard 17 -- a deviation that scholars have never fully explained.

Among the Reihokan's most intimate artifacts are two scrolls of the Sangosiiki -- a comparative study of Confucianism, Taoism, and Buddhism -- written in 797 by the 24-year-old monk Kukai before he had traveled to China, before he had founded Shingon Buddhism, before he had become Kobo Daishi. The text argues for the superiority of Buddhist practice, and the handwriting is unmistakably that of a young man making his case with forceful confidence. The scrolls total 39 pages. To hold a document written by the founder of the tradition that has defined this mountain for twelve centuries is to collapse the distance between past and present in a way that few museums can offer.

The collection extends well beyond paper and silk. Six wooden sculptures carved by Unkei in 1197 during the Kamakura period -- rendered in colored hinoki cypress with crystal eyes -- rank among the museum's designated National Treasures. A twelfth-century lacquer chest, covered in black lacquer with gold-dust images of plovers in a marsh, is the Reihokan's sole crafts-category National Treasure and was likely used to store sutras. Perhaps the most unexpected holdings are Japanese manuscript copies of the Wenguan Cilin, a Tang dynasty imperial poetry anthology. The Chinese originals were lost as early as the ninth century; the Koyasan copies, dating from the seventh through ninth centuries, are among the only surviving records of this once-celebrated collection. A large archive of ancient documents spanning the Heian through Azuchi-Momoyama periods preserves letters from Minamoto no Yoritomo, Minamoto no Yoshitsune, and the poet-monk Saigyo Hoshi.