Because previously most people had belonged to the one Catholic Church, and so religion was not a cause of wars or disturbances. It was something most of the European populations held in common.
One excludes the Albigensian revolt in southern France, and the Hussite revolt in Bohemia, which both led to civil wars.
The fact that everyone was Catholic didn't stop kings from going to war against each other out of territorial greed or whatever.
After the “Reformation”, people were divided on the most fundamental issues of life and salvation: Catholics, Lutherans, Calvinists, Anabaptists, Anglicans, Presbyterians.
Religion now became a major component in hostilities between nations and within nations.
There was little idea of individual choice of religion. Catholic Christianity had for centuries been a corporate, national faith, the backbone of society and all social life.
When rulers espoused the new Protestant doctrines, they tried to force them on the entire society they ruled, and often persecuted Catholics. Catholic rulers tried to prevent Lutheran doctrines infecting their people, and reciprocated with persecution of Protestants.
So instead of harmony, there was now conflict; instead of unity, division; instead of peaceful coexistence, persecution and war. The troubles continue today.
