FALSE
There was warfare already within the Holy Roman Empire during the Protestant Reformation. Martin Luther's ideas played a role in sparking the Peasants' War in Germany (1524-25), as peasants revolted against nobles and landlords. The peasants took Luther's reform movement against the church as a signal for changing society as a whole -- which was not something Luther himself supported. The nobles put down the peasants' revolts, but it was definitely not a peaceful time.
Immediately following Luther's death, war broke out between the forces of the Holy Roman Emperor, Charles V, and Lutheran princes. Charles V went to war against the Lutheran princes in the Smalcald Wars (1546-57) to try to force them back into adherence to Catholicism. There were ongoing struggles in the years after that, and ultimately, the Thirty Years War (1618-1648) was waged in the German territories (aka the Holy Roman Empire) over religious and other issues.