You obviously don't understand your own God.
Wanna duel over it?
Sartanism's willingness to shake things up is what makes it fun, however it needs to be done intelligently. Caspius' apparent policy of "attack first, think later" may have been fun for bored Sorrainian veterans, it wasn't for Sartanians outside the theocracy who desired some kind of political career. What makes a religion fun is successful conflict, and part of that is being a good diplomat to allow you to get away with religious wars on a continent where secularism is the ruling ideology.
You're assuming Caspius was going to lose his conflicts, and that he was a poor diplomat. To every man his opinions, but Caspius did manage to keep Sorraine in a good position whilst being alive.