Any way to sour the relationship between another character and the pope? Playing as the king of Bohemia (independent from the HRE) I'm married to the queen of a very large and strong Ruthania, and our first born son is set to inherit both kingdoms...and turned out really well (Strong, 17 in both martial and Stewardship). 

I don't want my realm seperated, so I set my sights on Poland...figuring I can do an invasion before my ruler dies (since I'm slightly smaller than Poland and have more than enough piety) and have enough money and allies to win the war. The only thing stopping me is the pope. I've done everything I can to butt-kiss the pope...but he still won't grant the invasion. Is there any way to sour the relation between him and the king of Poland?