The Polans were a West Slavic tribe that lived in Greater Poland. Polans united other Polish tribes and created Poland. The first Catholic ruler of the Polans was Mieszko I. Here is an extract of Helmold's Chronica Slavorum:
"Slavorum igitur populi multi sunt habitantes in litore Balthici maris. Sinus huius maris ab occidentali occeano orientem versus porrigitur. Appellatur ideo Balthicus (...) idemque mare barbarum seu pelagus Schiticum vocatur a gentibus, quas alluit, barbaris. Hoc mare multae circumsedent naciones. Dani siquidem ac Sueones, quos Northmannos vocamus, septentrionale litus et omnes in eo continent insulas. At litus australe Slavorum incolunt nationes, quorum ab oriente primi sunt Ruci, deinde Polani, habentes a septentrione Pruzos, ab austro Boemos et eos qui dicuntur Marahi sive Karinthi atque Sorabi."
in English, it means:
"There are a lot of Slavic tribes, that live on the coast of the Baltic Sea. The bay of this sea is directed from the west ocean to east. It's called Baltic (...) This sea is also called barbarian or Scythian from the barbarian tribes, that is failing. There are a lot of tribes that live around this sea; namely Danes and Swedes, which we call Normans, they live on the northern coast and all of the isles. Southern coast inhabit Glavic tribes, the first of them are Rus'; there are also Polans, that border in the north with Prussians, on the south with Czechs, and these, that are calling themselves Moravians or Carynthians and Sorbians."