What if Adalbert of Prague was successful in converting the Baltic Prussians and was not martyred, how would that affect the history of the Baltics?
If the Old Prussians were converted int he 990s, then Prussia would have entered the community of Christendom as Poland (966) and Hungary (1000) did around that time. The leading Prussian lord would probably receive a crown and become King of Prussia. Its development would probably be much like Poland and be heavily influenced by German customs and town law.
After consolidating power and establishing a real state, Prussia would probably attempt to conquer and convert the rest of the Baltic tribes, particularly the Lithuanians and Latvians. If there were ever a point there would be a northern Crusade, any military orders would likely be subservient to the Crown of Prussia. Most likely the end result would be a unified Christian kingdom of the Balts. This would probably accelerate the spread of Latin Christianity in the region by a century or two.
It is interesting to speculate the impact of that in northwestern Russia. Given the huge importance of Lithuania in establishing the Polish-Lithuanian commonwealth, this has huge impact on the development of Belarus and Ukraine as well. If there is not an ongoing push by the Teutonic Knights to control the region, then that has huge impact on German history and ideas about the Drang Nach Osten.
They might end up joining with the Holy Roman Empire if they convert. This will allow Christianity to penetrate the Baltics even quicker and possibly weaken the social structures in Lithuania that were quite strong for Paganistic culture.
Prussians being involved early on might mean that they are more of a unique group compared to the rest of Germany as they wouldn't have to get grinded into nothing by the Teutonic Order.
They might end up joining with the Holy Roman Empire if they convert. This will allow Christianity to penetrate the Baltics even quicker and possibly weaken the social structures in Lithuania that were quite strong for Paganistic culture.
Prussians being involved early on might mean that they are more of a unique group compared to the rest of Germany as they wouldn't have to get grinded into nothing by the Teutonic Order.