Take him anyways
Respect his wishes
The Demon is clearly speaking through him and trying to avoid being cast out. Save the poor man by ridding him of the Demon.
It's for his own good, and as somebody else said, them saying no might have been the demon speaking.
Same reason as the other two before me.
Because... Blue Exorcist is a great character
To force someone to endure your crazy religious ceremony without their consent is assault. Just because you are doing it for religious reasons does not make it any less awful.