IIRC Bluetooth was the elected monarch of multiple tribes and it was he who made the danes christians or so it says on a rune stone... The "legend" goes that a monk came and tried to make them christians by "proving the power of God" by putting on a red hot gauntlet and come out unscathed...
Must have been a tough sell... Trying to convince bearded vikings that they can no longer pillage as they please, they dont go to valhalla during honorable combat, they cant have thralls nor drink etc etc etc... Must have been more to it... Maybe he knew that if they didnt convert the next crusade would be against the north instead of the middle east