Quote Originally Posted by Geezer57
For Inquisitor attacks on the Pope, the province needs good zeal, the Pope's piety should be fairly low, Bishops/Cardinals shouldn't be present (not a deal-breaker, but helpful), and your Inquisitor or GI should have lots of valor stars. If your target has some piety, don't just send one Inquisitor, send them in packs. When you trail for heresy repeatedly, the subject gains positive piety virtues at first, then gets the Atheist line of vices, which drop piety by -8. Your trial should succeed after that, unless you're very unlucky.
Aside from occasional military action, trying the Pope for heresy is my preferred method of dealing with him. A few Grand Inquisitors with decent valour are generally far more effective than flocks of assassins.