Oh we have great cause-we want to renew the church by scraping the charter, and because of the supposed corruption of the elders.
Except that you
never tried to actually change the charter in any way. Niselur said
nothing about the charter. Ever. Until one day Leopold suddenly stands up, with no warning, and screams "This crusade sucks! The charter sucks! The church sucks! WAR!!!!!" Let's face it, most of the people in your realm weren't even members of the church until after Leopold's rebellion. Lain wasn't even a member of the church until after the war was already planned, and set in motion.
If Niselur truly desired to renew the church and rework the charter, they would have actually attempted to do so by, you know, actually trying to do so before declaring war.
That and we have built up too much momentum to stop. If we backed out, our allies may never support us again, and then the church will crush us.
Which is actually a perfectly legitimate reason for going to war. All this "we want to renew the church and get a better charter" is just hokum.
When your character offered peace terms I think he underestimated the momentum behind this war, and the planning that went behind it.
We knew Leopold would refuse before we even sent the offer to him. Yet the offer had to be made, regardless.