Hate to say it but Vellos is right... Unless you are an ex-member that still has influential ties that is. I was surprised because after leaving the church I became much more motivated to make my own mark as a noble,born to lead let the peasants follow.
Hence why the church is starting to fail in Niselur. I may be new to the game, but from what my character saw, there was little engagement from SA when she arrived in the realm. On the other hand, King Leopold immediately engaged me in the realm, and any knights that arrived soon after, if the church did that to new characters, maybe things turn out differently and King Leopold's actions in Niselur would be viewed in a different light.
Before and after the crusade conflict I see no priests in Niselur preaching the faith, while my adventurer sees many in Luria Nova, which I think fosters a disconnect between the church and nobles of Niselur. In fact it was not until Leopold made his decision known to nobles of Niselur that regent Mordaunt began to engage the realm, and despite his skill at debating it was too late-loyalties were set.
Under King Leopold's reign nobles seemed to be allowed to speak out, which fostered an independent and blunt attitude amongst the nobles, ill suited to being told what to do by other powers.
This set the stage when Leopold announced that he was being considered for punishment by the elder council for the entire kingdom to be incensed that the King of all things was being told what to do, which hurt the national pride. Any conflicts of loyalty that members of SA in the kingdom of Niselur were shattered when they were punished for speaking out against the crusade(at least that is what they told my character).
Now, maybe the regent Mordaunt would have changed the opinion of nobles in Niselur with his skilled wall-of-text attacks, but then he attacked Phantaria completely disregarding the King's wishes that Niselur not participate in the crusade. This enraged the nobles further(and my character).
You would think that Mordaunt was finished souring people against the the church, but then on trial he proceeded to claim that oaths of fealty to the king meant nothing, and that the laws of the realm did not apply to someone so high up in the church-this caused your "I'm a simple solider(probably people who didn't have as much time to put in the game)" character types who normally wouldn't say anything and care nothing for politics to start speaking out(with some nobles saying that "I normally don't do this but... I'm outraged at the disrespect shown to the king").
Then after the trial, he ignored the judge's orders to apologize and started attacking the king publicly, by then nobles had started to tune him out.
King Leopold seems to have the solid support of Niselurian nobles behind him(unless there is some grand plot that my character is not aware of).