Did Christianity spread through trade?

There is historical and archaeological evidence that suggests trade was influential in the spread of Christianity. Early on, the official means for spreading the "good news" was through missionaries. Men like Paul of Tarsus went out into the world to preach publicly and inspire Jewish and non-Abrahamic peoples to convert. Initially, Christianity was a sect of Judaism, but Paul was significant in permanently separating the two faiths. Many early missionaries thought people should convert to Judaism and then to Christianity as a sect of Judaism, but Paul thought (and taught) this double conversion was unnecessary.

After Paul's journeys throughout the Mediterranean, the next biggest conversion to Christianity was when Emperor Constatine issued the Edict of Milan, which decriminalized Christianity. Constantine is upheld as the first Christian Roman Emperor, and he made it safe for people to practice Christianity in the Roman Empire without fear of death or punishment. 

Between Paul's time in the 1st century CE and Constantine's Edict of Milan in in 313 CE, Christianity was booming in the Roman Empire but had to be practiced almost entirely in secret. Trade played some part in the spread of Christianity, as the entire Roman Empire was connected by trade routes. The only people really likely to be traveling were involved in trade or were intentional missionaries. By the 9th century, Christianity spread as far north as Scandinavia. In Anders Winroth's The Age of the Vikings, he describes graves found in Scandinavia where people were buried with crosses that originated as far away as Turkey or Greece. Even after the fall of the Roman Empire in the west, trade routes connected Europe and exposed people to Christianity.

During the second millennium CE, trade expanded globally, with ships regularly sailing from Europe to Africa to Asia and even to the New World. Though the primary purpose of far-distance international trade and New World exploration was to acquire valuable goods, there was a significant secondary effect of evangelization. Especially in relations with East Asia, evangelization was on par with trade in terms of importance. With Europeans claiming territory in the New World, they could declare the land as belonging to a Christian nation and officially make anyone who lived there a Christian. By the time the Americas were colonized, Europeans came to see it as a duty to convert the native peoples to Christianity.

Christianity really flourished in the context of a world that was connected by trade, so much so that conversion has almost replaced trade interests in more recent centuries.