Year 1:Invade the weakest country on your border with a fortification, let some escape to the fort.

The pope will threaten you with excommunication.

Year 2:Withdraw your troops from the weak country and invade the other christian countries.

You will not be excommunicated as the pope can only give one warning at a time.

Year 9: Withdraw all your troops sieging an enemy fortification.

Year 11: Repeat. I don't know whether it is year 10 or 11 because I don't know wheter the pope threatens you in year 1 or 2, so withdraw at year 9 and invade at year 11.