Final answer:
Martin Luther authored the pamphlet 'Against the Robbing and Murdering Hordes of Peasants,' advocating for the suppression of the 1520s peasant rebellion to maintain social order and support his reform movement.
Step-by-step explanation:
The religious leader who urged the German princes to ruthlessly suppress the peasant rebellion of the 1520s in his pamphlet “Against the Robbing and Murdering Hordes of Peasants” was Martin Luther. He was a central figure in the Protestant Reformation and is well-known for his criticism of the Catholic Church's practices, such as the sale of indulgences. His movement indeed relied on the support of secular authorities, as he saw cooperation with them as essential to promote his religious reforms. The peasant rebellion threatened the social order which Luther believed should be maintained, leading to his harsh stance against the rebels.