Kereopa Te Rau was one of the five original disciples of Te Ua Haumēne, the founder of the Pai Mārire faith. He was a member of Ngāti Rangiwewehi of Te Arawa. The date and place of his birth are not known, nor the names of his parents. Some time in the 1840s he was baptised by the Catholic missionary Father Euloge Reignier, and took the name Kereopa (Cleophas). He is believed to have served as a policeman in Auckland in the 1850s. In the early 1860s he fought in the King's forces in Waikato. His wife and two daughters are thought to have been killed at Rangiaowhia, near Te Awamutu, when it was attacked by government forces on 21 February 1864, and the following day he was at Hairini, a defensive position just west of Rangiaowhia, where he saw his sister killed.
After the defeat of the King movement forces in mid 1864, Kereopa joined the new religion of Te Ua Haumēne. In December 1864 Te Ua instructed Kereopa and Pātara Raukatauri to go as emissaries to the tribes of the East Coast. They were told to preach the Pai Mārire faith in the districts they passed through, to go in peace and not to interfere with Pākehā. Kereopa, however, demanded that a European be given up to him at Ōtipa, a settlement on the lower Rangitaiki River, and that a Catholic priest be handed over at Whakatāne. These requests were refused, but at Ōpōtiki the missionary C. S. Völkner was seized and ritually killed on 2 March 1865. Völkner was hanged from a willow tree near his church by members of his own congregation, Te Whakatōhea. His body was then decapitated and Kereopa swallowed the eyes, calling one Parliament and the other the Queen and British law. Although this act outraged Europeans, such an indignity to the head of an enemy conferred mana on Kereopa.
Kereopa was widely believed to have instigated the killing of Völkner. Although he had agreed to it, in fact he did not take part in the actual hanging, and cannot be held responsible. The arrival of the Pai Mārire party at Ōpōtiki precipitated the tragedy, but there were complex reasons for Völkner's death. Principal among these was Te Whakatōhea's anger at the missionary for his actions in spying for the government; in returning to Ōpōtiki at that time Völkner had disregarded the explicit warnings of Te Whakatōhea. Kereopa himself may also have sought to avenge the deaths of members of his family at Hairini and at Rangiaowhia, a plan of which Völkner had sent to Governor George Grey.
After the killing of Völkner, Kereopa, with his party of Pai Mārire followers, went on to Gisborne, and to the Urewera where he preached the Pai Mārire faith among Tūhoe. In May 1865 he attempted to travel to Waikato to preach to the Kingite tribes, but was prevented from reaching the Kaingaroa plains by a force of Ngāti Manawa and Ngāti Rangitihi. According to one account, in the course of this battle, in which Kereopa's party was supported by Tūhoe, Kereopa swallowed the eyes of three Ngāti Manawa warriors who had been killed and decapitated; it was this repetition of his symbolic act at Ōpōtiki which earned him the name Kaiwhatu (the Eye-eater). After a long siege Ngāti Manawa and Ngāti Rangitihi abandoned their defences at Te Tāpiri and Ōkupu, in the western Urewera, but Kereopa was forced to turn back when a relief party of Te Arawa, led by W. G. Mair, arrived. He then returned to Ōpōtiki but was driven from there by government troops, and fled into the Urewera.
Kereopa had much mana in the eyes of Tūhoe, as the bearer of the Pai Mārire faith to that tribe, and thus obtained their protection. The dense bush of the Urewera mountains also offered him protection from his pursuers, as it later would for Te Kooti. Martial law had been declared in the Ōpōtiki and Whakatāne districts after the killing of Völkner, and a reward was offered for the capture of those responsible. Kereopa concealed himself at Te Roau, on a densely wooded hillside, Te Miromiro, at Ōhāua-te-rangi, a Ngāti Rongo settlement north of Ruatāhuna. Te Roau had never been occupied, and commanded an excellent view of anyone approaching. There Kereopa was able to elude his pursuers for the next five years.
From mid 1868 the Ringatū faith of Te Kooti gained popularity among Tūhoe, and the influence of Pai Mārire correspondingly faded. The reverence in which Tūhoe held Kereopa also diminished, but Tūhoe did not disclose his whereabouts. Over the next three years, however, the people of the Urewera were weakened, and their land devastated, by the government's relentless pursuit of Te Kooti and the remaining Hauhau leaders. Government troops, including a Ngāti Porou contingent led by Rāpata Wahawaha, embarked on several campaigns between May 1869 and early 1872, in which Tūhoe pā were plundered, crops destroyed and many people killed.
By late 1870 several Tūhoe leaders had made their peace with the government. But they would not violate the sanctuary of the Urewera by giving up Kereopa. Eventually, however, realising that their survival was threatened by Kereopa, they decided to withdraw their protection.
Tūhoe tradition gives the following account of the capture of Kereopa. It was agreed among Tūhoe that neither European soldiers nor Ngāti Porou forces should be allowed to capture the Hauhau leader; as his protectors, they would deliver him themselves to the government, to ensure that their own mana was retained. Thus a Tūhoe party went to Te Roau, in September 1871, and laid their plans before him. Kereopa agreed to give himself as payment for the Tūhoe blood that had been shed for him. When he went to gather his possessions from his sleeping house, however, he attempted to flee. He was chased and captured by a warrior named Te Whiu Maraki, and taken to Ruatāhuna. Because he had broken his word, he was handed over as a prisoner to Rāpata and Captain Thomas Porter.
On 21 December 1871 Kereopa stood trial at the Supreme Court at Napier for the murder of Völkner. There was no direct proof of his responsibility for the killing, but a European witness, Samuel Levy, testified that he had seen Kereopa among those who escorted Völkner to the willow tree. On the basis of this evidence Kereopa was convicted of murder and sentenced to death. William Colenso appealed unsuccessfully for clemency on the grounds that the crime had already been punished by executions and land confiscation. Mother Mary Aubert, of Father Reignier's mission at Napier, stayed with Kereopa during his last night. He was hanged on 5 January 1872 at Napier.
In 2014 a statutory pardon for Kereopa Te Rau was part of a Treaty of Waitangi settlement between the Crown and Ngāti Rangiwewehi.