When I played as the Holy Roman Empire, I found that the best diplomatic strategy is to say damn all the rest and bend over backwards to please the Pope, his friendship is the only deterrent to numerous attacks from your neighbors. The best way to make friends with the Pope is to pay him to be your friend. On top of trying to get experienced priests to become cardinals you must PAY HIM OFF. I realize this is hard to do as the HRE because funds are in short supply. To do this, whenever I have a particulary good turn or sack a settlement I always give a nice cut to the pope. If it is your pope or a friendly country's pope that is in power, you can offer tribute of about 4000 to get outstanding relations. Otherwise expect to pay 8-10k for the same result. The Pope loves money, pay him, be his friend.