as for why he called her "that thing" is because she is considered useless(that is if he knows that who she really is and is not a ruind noble, but since he is the prime minister I think he does). they don't know what her skill can really do and she's keeping it a secret. so, from their POV they made a lot of effort to summon people, got one priestess and one useless person that is just a burden(it would be better if she hadn't come - but not like she cound choose that ╮(￣▽￣)╭).
this is just my speculations, but eather they have some plans for the pristess like secrefice (they are the "bad guys") - highly unlikely; or maybe they just don't want mc to have any influence on priestesses (maybe because she could use her and couse there to be problems between pristeses and the people of this world) or just so the pristecess is isolated and she only relise on them).
sadly there is no novel, so we will just have to wait to find outヾ(❀╹◡╹)ﾉ~
SOOO... Why is she being kept away from the priestess and why did that weird glasses guy look at her like that and call her 'That Thing'