Quote Originally Posted by sbroadbent
If you can access them, I'd also consider building Inquisitors. Find your province with the highest Zeal rating, and move your king there. Train a bunch of Grand Inquisitors. The regular one's won't be able to to do the job.

After a couple attempts, your king should have a few VnV's such as Athiest, and have 0 Piety. You would need 0 Piety, 100% Zeal and a Grand Inquisitor to get at best a 25% chance of success.
Good covering, but a few notes. A high star grand inquisitor doesn't need to bother much about the piety of the king.
And to rise piety in a province use inquisitors. To make sure that they doesn't burn the population, make them busy with trials.