I happened to be in Saudi Arabia in 1991 while in the military. One of the Saudi air traffic controllers (British educated as it happens) once told me that if a Sunni and Shi'a were to meet on a road and Allah told each that the other was the last of his kind, they would fight to the death. How does one begin to make peace between them?
I'm pretty sure you could have told that same story a few hundred years ago with a protestant and a catholic. The first step is identifying with the problem and realizing that Christianity is really not all that different. If we look back at the past, we might be able to find some keys to unlocking a more peaceful future.
The difference is, no third party convinced the Protestants and Catholics to forget their enmity.