The Boxer rebellion was not a rebellion against Christian missionaries, it was a rebellion against all foreigners. The Chinese became xenophobic when the western powers started carving China up into spheres of influence, usually surrounding a port controlled by a European country.
I have said that an important underlying factor was anger at foreign influence. However, a very important manifestation of this influence was the presence of Christian missionaries in rural and small-town China, away from the Treaty Ports. The rights of missionaries to work and prosyletise in the interior of China was one which was won by treaty (secured under the threat of armed force), and missionaries enjoyed powers by treaty which rivalled those of their main enemies, the Confucian local gentry and magistrates.
Some grievances against missionaries were invented (missionaries and nuns running orphanages, for example, were frequently slandered by the accusation that they were taking abandoned children in for human sacrifice). Others had more substance: the building of western-style churches with spires was held to offend local geomancy and was, at best, culturally insensitive; even more significant was the attraction of the church to 'bad characters' (so-called 'rice Christians') who would seek the protection of Christian status, which was believed to bring with it the protection of foreign arms (and often did so). This is not to deny the genuine converts; but there were many false converts. The Boxer Rising was initially a rural affair, and their target was not the business magnates and foreign garrisons of the treaty ports; it was the foreigners they new and, sadly, distrusted: missionaries. Leatherman 16:52, 27 April 2007 (EDT)