To what extent did the Peace of Augsburg of 1555 introduce religious toleration in Europe?
The Peace of Augsburg was established September 25, 1555, when the Holy Roman Empire assembled for their diet, meaning conference, in Augsburg that year, 38 years after Martin Luther began his revolt against the Catholic Church by posting his Ninety-Five Theses to the door of the Castle Church in Wittenberg. The Diet established the treaty knowing that the religious friction would not come to an end without a peace settlement. However, Holy Roman Emperor Charles V was wholly against compromising with the Lutherans and refused to attend the meeting at Augsburg. It was up to the Diet, without Charles V, to forbid future rulers to go to war based on religion. The Diet also hoped that the peace established under the treaty would lead to reconciliation between the two churches ("Peace of Augsburg").
The Peace of Augsburg established religious tolerance for Lutheranism in Europe alongside Catholicism; however, the peace treaty of course did not remove all tensions. The treaty established that the ruler of any territory within the empire would determine which religion to follow. The citizens within the territory who preferred to follow the other religion would be obliged to move. There were also certain areas in which citizens were free to follow either Lutheranism or Catholicism within in the same area. The Augsburg City Council had to make violence due to religious causes illegal, making use of both interrogations and harsh sentencing to squash further offenders ("Scandal and Punishment: The Holy Roman Empire"). Nonetheless, there were of course still many civil wars and violent insurgences that took place after the treaty was enacted, lasting until 1598. For example in 1559, only 4 years after the treaty, French Calvinists started a civil war against their Catholic King, Henry II. Conflicts between French Protestants and Catholics continued due to issues arising in both 1562 and 1572. It's estimated that 70,000 Protestants were killed in 1572 between August and October alone ("Lecture 6: Europe in the Age of Religious Wars, 1560-1715").
Hence, even though the Peace of Augsburg established legal religious tolerance by recognizing Protestantism, naturally, it would take many, many more years before true peace and true tolerance finally reigned, resulting in still many more brutal deaths.