Actually before Christianization there was no Poland, Mieszko I created the Polish nation, two of the Lechitic people already accepted Christianity from Methodius and Cyril, the Silesians and the Krakovians aka Vistulans, if the Polans were never Christianized the Polish nation as we know it would never exist and Polans might be conquered by a Slavic nation that adopts Christianity first or by the Germans.
Yeah because Poland was a wasteland before 996. Poles suddenly appeared and formed a coherent territory when they were christianized.
Seriously, no. The notion of nation don't even exist during the all Middle-Ages, so christianisation HARDLY created it for western slavic peoples.
What he created was the Kingdom of Poland, the same way it created Moravia : it allowed former pagan people to enjoy recognition by other kingdoms. More or less like some countries search to be recognized in the UN.
Christianisation sanctions states, it doesn't create it (specifically for Poland, It was german clergy trying to put a buffer zone between Balts and Eastern Slavics and by making Miesko Amicus Imperium. (It backfired a little, as German clergy was soon replaced with Polish clergy)
For the Silesians, Lechitic, etc. Yes, Poland was full of different peoples. Like every part of medieval Europe. Searching an heterogen kingdom in Europe made by ONE people can take time : no one had that.
For France, Aquitains, Burgunds, Franks, Flemish, etc that kept an indentity quite far, up to the XVI where the idea of nation begin to exist.
Same in Poland, Masovians, Silesians kept a strong identity up to the concept of nations appears in the region.
Again, NO nations in Europe before XVI-XVIII, but peoples or kingdoms.
Admittedly, in the X century, the differenciation between western slavic peoples was achieved : thanks to imperial diplomacy (Carolingian or Ottonian), relativly undifferenced tribal federations or slavic kingdoms achieved some internal evolution.
More or less the same way balkanic slaves formed themselve thanks to their relations towards Byzantium (or HRE for Croats). Christianisation is only the achievement of the process, not the beggining. Like Francia pre-existed to the Clovis' baptism, Anglo-Saxons to missions, Saxons to their conversion or Danes before adopting Christianism.