From Emperor of Japan: Meiji and His World, 1852–1912, by Donald Keene (Columbia U. Press, 2005), Kindle pp. 137-138:
The young emperor’s first act of major historical significance was undoubtedly the promulgation of the Charter Oath in Five Articles on April 7, 1868. The swearing of this oath before the gods of heaven and earth, in the presence of “the hundred officials” including nobles and daimyos, was preceded a day earlier by an edict that announced the renewal of various ceremonies of Shintō worship after the extremely long hiatus imposed by the military regime. The stated purpose of the edict was to revive the union of rites and rule that had existed in ancient times.
A central element in the plan of restoration was the reestablishment of the Jingikan [神祇官], the Ministry of Shintō. It had originally been established at the beginning of the eighth century, but for centuries had possessed little more than vestigial significance. Now, however, Shintō priests and the performance of Shintō ceremonies at the court and at shrines were to be placed under its supervision, and the priests were to resume functions that had long been left to surrogates. The renewed importance of the Shintō priesthood and the insistence on separating Shintō from Buddhism were made more explicit four days later when Shintō priests who served concomitantly as Buddhist priests were ordered to yield their Buddhist ranks and positions, give up their Buddhist robes, and let their hair grow out.
For more than a thousand years, most Japanese had believed simultaneously in both Shintō and Buddhism despite the inherent contradictions between the two religions. For example, according to Shintō belief, the present world is lovely and a source of joy, but yomi [黄泉 lit. ‘yellow springs’], the world after death, is a place of foulness and corruption. According to Buddhist texts, on the contrary, this world (shaba [娑婆 lit. ‘old-woman old-woman’!]) is a place of trial and suffering, but one’s actions in this life can enable one to enjoy after death the joys of paradise. These and other fundamental differences were generally minimized by those who discussed religious matters. Instead, the doctrine of honji suijaku [本地垂迹 ‘original-land hanging-trace’], which explained the Shintō divinities as avatars in Japan of the eternal Buddhist divinities, was widely accepted. In keeping with the projected return to the system of religion and government that had prevailed in the time of Jimmu, the first emperor, Buddhism, a foreign religion, was now rejected and even persecuted.
Even during the long period when Buddhism played a far more prominent role in the state and emperors regularly entered Buddhist orders and were known posthumously by their “temple names” (in [院]), Shintō was never neglected by the imperial family. The most important rites performed by the emperor were those of Shintō, beginning each year with shihōhai [四方拝], the ceremony of worship of the four directions, carried out at four o’clock on the morning of New Year’s Day. The emperor prayed to the star under which he was born, to the gods of heaven and earth of the four directions, and to the tombs of his father and mother for abundant crops, a long reign, and peace in the realm—all benefits in this world, in keeping with Shintō’s this-worldly outlook. Mention of the star under which the emperor was born was an indication that the Shintō rituals had been greatly influenced by Taoism. The court was dependent on on’yōji [陰陽師], priests of yin and yang, for predictions by divination of good or bad fortune. No action of consequence was undertaken in the palace without consulting an on’yōji.
Japanese religious life at the commencement of the Meiji era included elements of Shintō, Buddhist, Taoist, and other beliefs as well as what might be called superstitions. The decision to accord special importance to Shintō, and especially to the Jingikan, was, of course, closely connected with the enhanced importance of the emperor, who, according to Shintō belief, stood at the apex of the world.
The ritual accompanying the emperor’s pronouncement of the Charter Oath was entirely Shintō.