"he joined the church and bought himself a Chinese missionary"

I.e., donated money to a missionary organization working in China. The American desire to convert the ‘heathen’ was a product of the country’s view of itself as a progressive force whose destiny it was to elevate cultures it viewed as spiritually and morally unenlightened. In the case of China, this verged on a national obsession. Initially the Protestant missionaries — or ‘foreign devils’, as the Chinese preferred to term them — were not well-received: in 1814 a law was passed condemning them to death and any converts to slavery. However, in the latter 19th and early 20th century they played a prominent role in westernizing China.