So Mary's just a tool sent to turn Souma and his kingdom into more tools of that particular faith. Good choice not to give them an inch of agreement for now. Very sly that they chose a woman who had attributes similar to his future wives to be the "saintess" and emissary. Dodged a bullet there. I'd say ultimately he should say no. Not worth the trouble. Better to keep a cordial relationship with the Empire than this upstart theocracy. Also Jeanne x Hakuya must continue. Gonna be hard for them if the Empire and the Kingdom suddenly become hostile because of the Holy Kingdom's machinations.
Weird place to make a vocaloid reference, but that's probably gonna be important at some point. I guess that brings up the question that if she's got such an artificial-seeming personality, is she even actually a real person? What are the chances she's like a finely crafted puppet being manipulated in a similar manner as his Poltergeist skill? Having him accept something that isn't even a living thing as one of his wives is gotta be one of the biggest con jobs they can pull on him.
Still not a fan of harems in general, but I acknowledge this as a true harem where there are no harem collapses and no losing heroines. All 4 ladies will get a piece of him. I respect that he tries to make all 4 girls feel wanted and nobody is ignored. That was a nice hug, for what it's worth. I still root for Liscia though. First wife, best girl.