After leaving the seminary halfway through his studies, he became a pupil teacher at Mamfe near
Akropong. His heart condition interrupted his work and he was assigned to a less strenuous role as a catechist at
Larteh, south of Akropong. He found out at Larteh that the locals were vehemently opposed to Christianity and wanted a European missionary stationed there instead of an Akropong native due to the socio-political environment at the time. Thus, he returned to his hometown of Akropong. He was forcibly bound to palm fronds and taken to the porch of the Basel missionary, Johann G. Widmann. However, he was soon back at Larteh to support and run the operations of the mission there. This time, the people of Larteh were more receptive to the mission. According to historical accounts, he correctly predicted that a thunderstorm could lead to the fall of a silk cotton tree,
"onyaa" near the shrine of the fetish priest at Akonedi in
Larteh. The fetish priest rebuffed Opoku, stating that the tree had been there for centuries and never killed his ancestors. This event happened as foretold by Opoku and the fetish priest was killed. The people came to regard him as a “seer” and converted en masse to Christianity leading to the flourishing of his ministry. In 1868, while Opoku was in the Akuapem district as a catechist, two Christian converts killed an animal species, considered the “spiritual child” of the local divinity of
“Topere.” This escalated to a dispute which could not be resolved in the royal court. Opoku refused to pay any fines, asserting the authority of Christianity and God and stating that the idols could punish perpetrators on their own without any human intervention. Infuriated by his utterances, traditional authorities prohibited Christians from using the town's resources to fetch water or grow food. Christians had to go to other towns such as Amanokrom for food supplies or water. After a few weeks later, Opoku petitioned the chief to fetch water from the palace on humanitarian grounds. The palace obliged and the ban was lifted. Nonetheless, a decree was issued in 1869 that banned Mamfe natives from converting to Christianity. Parents of children who converted faced a monetary penalty, to be paid in cowries. Two girls, Kaade (c. 1845 – 1928) and Gyamea defied the edict and were baptized in March 1869 by Opoku, adopting the
Judeo-Christian Germanic names, Wilhelmine and Maria respectively. This led to near skirmishes, the capture and persecution of the two converts until the British colonial Governor intervened in the matter to free the Christian girl-captives. On 1 September 1872, Theophilus Opoku was ordained a minister of the Basel mission by the Johann G. Widmann together with the Gold Coast historian,
Carl Christian Reindorf and Jamaican Moravian missionary,
Alexander Worthy Clerk. Through his Christian ministry, he went to many towns and villages including a visit to the Togoland and the Northern Territories of the Gold Coast in 1877. He carried out ethnographic research in Salaga and his observations were captured in his diary which were ultimately published in the
Christian Messenger in
Basel in 1884. His accounts include everyday life of the Gonja people, the practice of Islam and the trans-Saharan slave trade. Within this period, the German ethnologist,
Gustav Nachtigal (1834 –1885) also travelled extensively in that areas with detailed description of the culture of the inhabitants of northern Ghana. After he returned from Salaga, he contracted smallpox. During his sickness, he composed a Christian hymn,
“Ohoho ne mamfrani na meye wo fam ha” meaning
"I am a stranger and sojourner in this world", a song which is sung at Presbyterian funerals in Ghana and is allusion to his journey to Salaga. In 1877, he was transferred to
Kukurantumi in
Akyem Abuakwa after recovering from the disease. In 1884, he was posted to
Adukrom, north of
Akropong and to Mamfe in 1891. He won many Christian converts from heathenism in both places through his mastery of his native tongue,
Guan language. His last station was his hometown,
Akropong in the Twi District in 1899. There, he faced opposition, from his own
kith and kin after criticised the lifestyles of natives, leading the traditional authorities to summon him before the royal tribunal. In 1909, the Local Committee of the
Basel Mission held a meeting at
Aburi and appointed Opoku a member of the committee, making him the first African to serve in that role. However, he declined the offer due to ill-health and impending retirement. He retired from active church work in 1911 at the age of 69. == Literary work ==