As I understand, after the defeat of a pagan army it was standard practice to force mass baptism as part of the concessions. While this would undoubtedly be deeply humiliating and perhaps even psychologically scarring to the defeated, a common refrain is that it was not a particularly effective way to create Christians--an account I just bumped into is that as soon as John of Bohemia and the Tuetonic Order departed after defeating and forcing the baptism of Lithuanan pagans after Medvegalis, the Lithuanians immediately returned to their old religious rites, because after all, why wouldn't they.^1
So was there actual belief among Christians at the time that even coerced baptism would cause the scales to fall from their eyes? Was it a diplomatic fiction, as Christianity was the way different princes related to each other? Was it pure propaganda? For the last one, I remember Eric Christianson's Northern Crusades describes a certain "gap year" quality to Christian knights going on Crusade in the Baltics (one modern popular historian amusingly described it as the Medieval equivalent of "doing it for the gram") and that labeling a group "apostates" who had rejected Christ allowed a freer hand than if they were pagans who simply did not know Christ, and so forced baptism could paradoxically create a pretext for further military action.
^1 I am drawing a distinction here between the narrow act of forced baptism and the actual demonstration of the power of one's god through victory in battle, which seems to have been a quite effective driver of conversion.
Hi friend,
About the global practice of mass baptism I couldn't bring much on the table. However, the event your referring to with John the Blind is something that rang my ear quite a bit.
As I've written before on this subreddit, the Reisen were a bloody business:
The Reisen were mostly held in winter, when war came to a stop in Western Europe but when the land was frozen enough for knights to venture through the marshes of Lithuania and Eastern Poland with their heavy armors and their many horses. A mild winter would result in the cancellation of a Reisen! It was also another type of warfare, against non-Christians. Everything was permitted against such foes. The rules of chivarly didn't apply and knights could really give it a go and still maintain their honour intact. This was very attractive beyond the promises of loot and plunder.
About the very mass baptism you're referring to, Tomas Borosvky wrote the following in a 2013 article:
Precisely because of the Silesian matter, John spent a relatively long rime at the turn of the year in Wroclaw, from which he headed for the centre of the Order of the Teutonic Knights in Torun. Here, he awaited information on the order army under the leadership of Grand Master Wernher von Orseln in Königsberg, for which John set out immediately and the two groups joined still in January 1329. Their attack aimed at Samogitia, where in a quick attack they conquered the fortified settlement Medelvagen on Groundhog's Day (1 February), where John supposedly had several thousand prisoners forcibly baptised, by which he however saved their life, because the Grand Master and his knights wanted to slaughter them.
According to the footnotes that goes with that piece of information, not much has been written about this event. He compares the various narrative sources that either put 6,000 or 10,000 baptised pagans. However, we need to appreciate how Borovsky wrote "John supposedly had several thousand prisoners forcibly baptised." Though he's not openly denying the fact he takes it with a grain of salt. Most of the narratives sources were actually biased and/or largely in favor of John the Blind. Guillaume de Machaut, the French poet--and very first French author to be called "poet" like the authors of old!--who accompanied him everywhere, took the event as an additional evidence that John the Blind was the ultimate Christian king. He may have exaggerated things in the process.
What was John the Blind hoping? How could we actually know? He may have tried to save the lives of the pagan warriors he just defeated out of political pragmatism. Maybe he genuinely thought they would convert to catholicism. Certainly, more research should be conducted on the matter.
I'm sorry I cannot help more. Maybe the answer lies in the latest biography of John the Blind? However it seems it is written in Czech and I couldn't possibly understand it...