Im sure that many questions on the spread of Christianity have been answered, but I am wondering about the interpersonal aspect. Why did missionaries have such an easy time with so many different cultures? One would think that most people would not be open to foreign strangers telling you your entire religion and way of life is false. What encouraged so many kings and other leaders to convert their kingdoms to Christianity? Obviously, the Roman empire helped spread Christianity to most of mainland Europe. But for instance the Norse, the middle eastern and Asian countries were mostly converted by missionaries. Countries with deeply entrenched religious beliefs all converted no matter how individual or strong those beliefs. I understand that the process wasn't fast or easy everywhere, but missionaries are received with enough sympathy that I have wondered this for a long time.
If this aspect of this question has been answered somewhere else I apologize, please let me know.
Thank you.
Excellent question! I've researched this subject in the past, I might be a bit rusty now, but I'm going to dig out some of my old sources and essays. This is my first time posting here, so I hope this response is up to the sub's standards. It's late here, so I've only been able to write the first half of my answer before bed. This first part doesn't answer your question exactly, but it discusses the difficulties of conversion to Christianity and how successful mission was achieved. I will return to finish the second part tomorrow, which will hopefully answer your question from the perspective of why pagans would want to become Christian willingly, rather than the role of coercion alone.
First, I have a couple of qualifications to make. As you note yourself, the process wasn't fast or easy everywhere. This is very important. The process of conversion, in terms of success, risks, speed, theology, differed across time and place. How the Roman Empire was converted in the 4th century was very different from the Christian mission to Scandinavia during and after the 800s, which was in turn very different from the conversion of Mexico in the 1500s, to the (unsuccessful) missions in Japan, Africa in the 1800s, and so on. I only feel qualified to speak about the conversion of Frisia, Saxony and Scandinavia (and particularly Scandinavia at that, and so my answer will reflect that.
Part A - Was it so easy to convert people to Christianity in Northern Europe, c. 600-800? And how was it done?
The second qualification is, can the process which saw the conversion of people to Christianity, at least in Northern Europe from the 600s onward, be described as "easy"? I would argue no. The Hewald Brothers were slain during the mission to Saxony in 692. Over half a century later, St. Boniface, perhaps the most famous of these missionaries to Northern Europe, was killed while undertaking his Christian mission in Frisia. He was most likely killed in an act of robbery, rather than an act of martyrdom (as his death is traditionally portrayed), but it still underscores the dangers faced by these Christian missionaries, operating beyond the borders of Christian kings (we will return to this point below, since it is a crucial reason for the success of later missions).
The image we have here is not one of success or ease, but one of failure. Frisia, as remarked by Alcuin in the Vita Willibrordi, about Willibrord, Boniface's predecessor as missionary in Frisia, was not successfully converted to Christianity until after the Frisian king, Radbod (d. 719), was brought to heel by Frankish arms. And as we saw above, this state of affairs was only temporary (Boniface was killed in 754). Saxony (and Frisia) would not be converted until after Charlemagne's conquest during the Saxon Wars (777-804), over a century after the Hewald Brothers, and the employment of whatever primitive (compared to now, at least, it was probably the most advanced in that region since the collapse of Roman government) apparatus of state existed under the Frankish realm in this period. For this, the Saxon Capitulary (c. 775-790) is a good source. This made cremation of the dead, sacrifice to pagan gods and prayer before groves capital offences, while similarly enforcing baptism and the abstention from meat during Lent and from labour on Sundays. Through force of law, pagan practices were disincentivised and Christian behaviour was encouraged, aiding missionary endeavour. Further, the introduction of tithes provided missionaries with a source of income to sustain their ventures, while the murder of churchmen was made a capital offence, shielding them from harm.
Yet, one wonders whether secular rulers in this period were even able to enforce such laws? Did the limited state apparatus of the Carolingian Empire allow for such policing? There is even some evidence to suggest that such legislation, rather than aid missionaries, in fact endangered them, as it provoked rebellion. This can be seen in a letter by Alcuin to Megenfrid, Charlemagne's treasurer, which suggests that, though these laws were enforced, they were actually detrimental to Christian conversion. That the Saxon Capitulary was made less taxing in response is perhaps recognition of this, but this was a modification, not a rescission. It lessened some of the effects of these laws, but secular power still intervened in Saxony on behalf of mission.
More convincing evidence for the dependency of missionary success on secular support comes from tracing the flows of Carolingian conquest and Christian mission in Saxony. Richard Fletcher writes that Saxony was converted 'against a backdrop of massacres and deportations, of missionaries killed and churches gutted, of village communities disrupted and demoralized.' Before Charlemagne, missionaries, like the Hewald brothers above, failed to gain traction among the Saxons. After Charlemagne's conquests, missionaries operated with more freedom. When Charlemagne was defeated at Roncesvalles in 778, the Saxons rebelled and missionaries fled. When Charlemagne reasserted dominion over Saxony, the missionaries returned. Missionary success in Saxony was thus tied to the violence of secular Carolingian power. Another source, the General Capitulary of the Missi (802), which promulgated obligations of vassals in regards to taxes, levies, summons during war, also includes this line on religion: That no one shall presume to rob or do any injury fraudulently to the churches of God or widows or orphans or pilgrims; for the lord emperor himself, after God and His saints, has constituted himself their protector and defender. Good political administration of the Carolingian Empire was seen as inseparable from its good religious administration, and slights against Christianity were seen as slights against Charlemagne (perhaps worthy of note, referred to in that source as 'the most serene and most Christian lord emperor Charles).
This above section shows that the spread of Christianity through Christianity was not an easy process. It took place over centuries, with the deaths of many missionaries and priests, Christians and pagans. It also demonstrates the importance of secular support to the success of religious mission, particularly the ability of Christian kings to coerce the religious affiliation of newly-conquered peoples. When they were operating beyond the borders of the Frankish realm, without the protection of the Frankish king, they enjoyed little, if any, success. On the flip side, when the Frankish realm was in the ascendant, Christianity accompanied Carolingian conquest.
Continued below
You may be interested in a question I asked previously, For what reason would members of feudal Japanese society convert to Catholicism?, which received an answer from u/Morricane that discusses the appeal of conversion from the Japanese PoV.