The whole island looks down on Sorraine because they've earned it. The realm's population, whether be it by law or not is irrelevant, is openly hostile to the other faiths of the island. That is public perception. The CoS is the only religion on the FEI that doesn't play nice with everyone else. Nobody else closes down or sacks temples, uses their religion to attempt to undermine governments, uses secular wars as an excuse to go burning temples, etc etc. They have earned their reputation. Even if they hadn't, attempting to convince everyone else on the forum is a pretty dumb way to go about it. If Sorraine is interested in the goodwill of everyone else, they should attempt to garner goodwill IG
To be fair, the fact that Sartanism doesn't "play nice" with everyone is part of what makes it so fun. Religions like MAE might be well respected, but they are also dead boring and to a large extent pointless as the very reason why they are well liked is because they don't actually do anything significant. Also the Church itself isn't as looked down upon as you make out; it has a significant presence in nearly every northern realm and no ruler has yet attempted to ban or destroy its temples (though if the Church dosn't get its diplomatic ability back on track then that could change).
However you are quite right in that the downside to this is that they shouldn't be surprised when their theocracy is looked down upon. If you're going to have a realm that disrespects the religions of its neighbors, then you can't expect your neighbors to rush to your aid when a pseudo-religious war goes bad (I say pseudo-religious because even if Sorraine didn't intend for it to be religious, its certainly viewed as such by many due to their lack of a coherent justification for the conflict).
