These holy wars usually start with the best on intentions - to make thing safer for a particular religion. They never stay that way. All such wars regardless of what they are called, who started it or why, all degenerate into bloody battles interspersed with massacres, looting and whole scale genocide. This always leads to reprisals and more fighting and genocide.
One typical example is the Crusade of 539. In 538 A powerful bishop declared Marigund a land of heretics (read Rebuilders) and pagans (read Lightbringers) and should be cleansed. In 539 an army of around 10,000 invaded Marigund. Within a month they had destroyed over 100 towns and villages, killing most of the inhabitants in the process. The fact that less then a third of people in those places were Rebuilders or Lightbringers was a details lost to them. They were cleansing the countryside for their faith. This quickly alienated ALL the Followers in Marigund who rallied to the King out of sheer necessity. When the invading army was defeated at the battle of Bratislau the Marigund showed them no mercy. They hunted down every invader they could find and took few prisoners. Less then 3000 survivors managed to struggle back to safety. Then the Marigund army crossed the borders and laid waste to everything they could find. The fighting only ended when the Duke of the Outer Midlands forced a peace on all the sides. Only then did the armies march home. Since then the border has never truly settled down in spite of the Dukes demands for peace. Raids and counter raids have gone back and forth, and rarely a year passes without some sort of battle (usually small) being fought.
Rumor has it the King sent the Bishops severed head to the Patriarch with the message to leave well enough alone. The truth of the tale is impossible to confirm of deny. What is true is that the bishops family had to pay an immense ransom retrieve his body. Also when the body was returned the king sent the corpse back in dung cart. A full dung cart. Common courtesy prevents me from repeating the contents of the letter that went with the body.
That example highlights an Ecclesia Crusade but both the Rebuilders and Lightbringers have launched many of their own over the years that were equally bloody and vicious. They seem to be an unwelcome but growing problem in the Midlands that are sure to bring more pain and misery to all involved.