The Ásatrú faith gives piety through human sacrifice. Pick a faction with the Ásatrú faith, like Sweden (Duke Björn Ironside of Uppland). Declare "Raid for Captives against [...]" and start capturing cities, or simply raid and execute everyone you find. Once you can enforce your demands, you'll get an additional few prisoners. Execute them all. That's how I went from the lowest Piety level to the highest in 3 or so in-game years.
This will also give you 100 dread.