Why did Philip believe that the religious question was also political?

Philip, Landgrave of Hesse, was deeply convinced that the religious question was inseparable from politics because he recognized that the survival of the new faith depended entirely on the collective security of its adherents. He understood that Protestant sovereigns and towns needed to unite in a defensive alliance to remain unassailable against the Roman Catholic house of Habsburg, whose emperor, Charles V, sought European predominance and the eradication of the Lutheran movement.12

Philip’s political strategy was driven by the fear that if the emperor triumphed in his broader struggle for power, the position of the Protestants would inevitably deteriorate. Consequently, he viewed the formation of a powerful, unified Protestant front—which eventually manifested as the Schmalkaldic League—as the only way to secure the freedom of the new faith and allow Protestant rulers to establish their own state churches. By linking religious reform to territorial defense, Philip transformed the Protestant movement into a significant political and military force within the empire.213