Based on what he has heard, the Church of Sartan explicitly advocates converting all nonbelievers by the sword. In other words, as long as it is allowed to exist with that as one of its tenets, it will always try to take over the continent.
Whoever told your character that was deranged or had an axe to grind. The only realm Sartanists want to "convert" would be the Way of the Dragon of Ohnar West. And as far as my character understands, the only reason why they were considered evil is the the first place was because they were originally the enemy of Arcaea. So you see the irony of attacking the enemy of Arcaea, only to have Arcaea side with them against their former allies....
Except that you (assuming you're Eva—sorry, I don't always know which forum names match which families ) flat-out lied to the entire continent about your intentions. Ingall asked Eva when Coralynth's forces were first sighted if they were planning to attack, and Eva said no. I don't recall the exact wording at this point, but it was pretty unequivocal.
Except that, in this case the exact wording is pretty important. It didn't help that Velax interpreted he exchange in exactly the same way you did, which happens to be completely inaccurate. The Taikun told everyone on the Ruler channel that Sasrhas was being attacked, but we had already ordered a retreat, due to our bungled coordination. And there was a real question of whether or not we would attack, my marshal specifically advised me not to, in fact.
So I was not lying or being deceptive when I said that no attack had taken place. Clearly though, responding to the Taikun at all was a mistake, because it became the thing everyone could point to and say, "Coralynth is horrible!" But when Emperor Velax calls you a liar on the ruler channel, it doesn't really matter whether you actually are. It was also clear that at least partly he was doing this for OOC reasons in order to shake things up a bit.
What the Emperor did not expect was that we would take his betrayal so personally. We went from being his most ardent friends and supporters to... "!@#$ the Emperor" (to borrow a phrase from Sandor Clegane).
And my character told the Emperor straight up, if he kept going down this path it would spell the Doom of Arcaea... I guess we'll have to wait and see whether Eva was a prophet or that it was just wishful thinking. But Arcaea has had a LOT of issues ever since.