From Yangtze: Nature, History, and the River, by Lyman P. Van Slyke (Stanford Alumni Assn., 1988), pp. 153-154:
In the second treaty settlement [after the 1856–1860 Opium War], prohibitions to inland travel were removed, and Chinese authorities were made responsible for the safety of such travelers. Ten additional treaty ports were opened to trade, including Nanking, Hankow, and two other Yangtze River ports. It was further stipulated that since foreigners might reside in such treaty ports, the powers would have right of gunboat as well as commercial navigation on inland waters. Moreover, the country was opened to missionaries, who were now permitted to travel at will throughout the empire, and to be at all times protected by the Chinese government—a provision often impossible to enforce against popular anti-Christian sentiment. Missionary cases, usually called “outrages” by the foreign community, were enormously troublesome throughout the nineteenth century. The French, presenting themselves in the 1860s as the protectors of Catholicism in China (despite anti-Catholic measures at home) and insisting that the Chinese government not establish direct relations with the Vatican, also demanded that the Chinese government permit the Catholic church to own property and to guarantee the return of all property that had ever belonged to it, referring specifically to those missions that had been established by Matteo Ricci and his successors in the seventeenth and eighteenth centuries.
Until the second treaty settlement, the Catholic church in China had maintained a tenuous but stubborn toehold as an illegal, underground religion. It had been proscribed in 1724 by the Yung-cheng Emperor, except for a few authorized clerics in imperial service at Peking. At this time there were roughly 300,000 converts in China, declining by the end of the century by perhaps half or two-thirds, served by forty or so foreign clerics and twice that number of Chinese priests. Despite the risks, religious orders continued to smuggle priests into China and to smuggle a few Chinese out for training and ordination. Foreign priests had to be secreted at all times, usually in the homes of believers, going out only at night or in covered sedan chairs or boats. This was a harsh and dangerous business. If discovered, foreign priests might be attacked by hostile mobs or bandits. Official punishment might range from deportation to imprisonment to execution. Chinese Catholics often came in for even severer treatment.
The most active mission arena was the southwest, comprised of Szechwan, Yunnan, and Kweichow provinces, where vicariates apostolic had long existed in Chungking and Chengtu, both under the French Société des Missions Ètrangères. Rough estimates—the only ones available—suggest that in the early nineteenth century, there were perhaps 70,000 Chinese Catholics in these three provinces. This region was far enough removed from Peking so that the prohibitions rested a bit more lightly there than in the eastern provinces; but by the same token, the protections of the second treaty settlement were less well-known and enforced. Although some Chinese Catholics had renounced their faith, as directed by imperial edict, many others remained loyal despite repeated persecution.
Against this background, a few Westerners embarked upon explorations of the Yangtze River, and their books began to appear before a curious public. These explorers were not, of course, the first nineteenth-century Europeans to travel on the Yangtze River. In 1841–1842, and Anglo-French naval force had penetrated far enough to blockade the Grand Canal, thus demonstrating the capacity to strangle the capital by preventing vital grain shipment, and to take Nanking under its guns. There the first of the Unequal Treaties, the Treaty of Nanking, was concluded in 1842. A decade later, during the T’ai-p’ing Rebellion, several Europeans visited the dissident capital at Nanking, and left behind fascinating accounts of their experiences. But these men had little interest in the Yangtze River itself, except as a means of access to the interior. Even more reticent were the Catholic missionaries who began to take advantage of the concessions wrung from the second treaty settlement but tried to remain invisible, like their illegal predecessors, by wearing native dress and going concealed in sedan chairs or boats.