How does Voltaire use Candide to clarify his beliefs on the futility of war or agression between protestant and catholic believers.
Voltaire views war as atrocious and anything but heroic. Candide is forced to fight against his will for the Bulgarian king, and his treatment by the soldiers is brutal. His "fellow heroes" treat him more like an enemy than a noble soldier.
You can read about this in Chapter III of Candide here on enotes at the link below. In this chapter, Voltaire describes the glories of war, the well-trained troops, inspiring music and brutal treatment from which Candide tries to hide. While all of this brutality is taking place, the kings are praying and having "Te Deums" sung (these are Catholic hymns of praise). Very ironic, having prayers and attempting to appear "holy" in the midst of brutality. But, there is more.............read this chapter and you will see.
Also, see the discussion of themes here on enotes. War and the futility of war is one of the major themes in the book, as well as religion. Voltaire believed that religion should be more like faith between man and God, and this is revealed throughout Candide where religious people are presented in the worst possible light, for the most part.