At the vety least, the Chantry has a good track record for what they can find. They seem to be correst that Andraste was more than just some spell-slinging tart, that theDarkspawn are basically just cursed due to the arrogance and corruption of some Tevinter wizards, and a couple other things. You can argue over the existence of a Deity still, but presumably something unimaginably powerful somehow built the Golden City.
Further, Corypheus isn’t a character who can be trusted to say the truth, not even from his own PoV.