One thing that I think helps is contact with areas outside the Union. That always opens up the chance of running into something that forces re-evaluation. Two of them that I know of (Macet and Berat) have had prior dealings with humans, before the Thirteenth Order. Rebek, as I mentioned before, is a religious dissident, so undoubtedly when she got old enough to be able to restrain herself and not just blurt it out, she would've been exposed to other ideas besides just what was state-sanctioned--and chose belief in Oralius.
As for Speros...what makes him tick is more complicated--and I don't know that I really want to spoil it. Though if anyone feels like guessing, they can PM me.