I read in the manual that...

[Manstein]

So, I decided to read the manual.

I read that you can change the religion of your faction to some other religion!

Is this possible?

[the tokai]

It is only possible with the vikings in the viking campaign. It happens when 80% of your people are catholic. But it isn't really a good thing because you'll loose the raiding bonusses that the vikings get and you wont be able to ship units from provinces without a port.

[Ludens]

It is only possible with the vikings in the viking campaign. It happens when 80% of your people are catholic. But it isn't really a good thing because you'll loose the raiding bonusses that the vikings get and you wont be able to ship units from provinces without a port.
But the good part is you can do it to the Vikings when playing another faction, putting an end to the endless hit-and-run attacks of the Vikings. Though, I never managed to do this: my non-Viking VI campaigns always end quite quickly due to civil wars, lack of money and kings without offspring.