Wang Tao currently cannot verify the truth of the Bishop's words, but Wang Tao's spiritual power is akin to a lie detector, and he can perceive that the Bishop is not lying about wanting to save all of humanity. His belief is extraordinarily firm, as if as long as he acts, he genuinely can save humanity.
Wang Tao doesn't know why he's so confident; perhaps he's had a fortuitous encounter like himself, possessing some sort of external aid not to be shared with outsiders?
Even if everything the Bishop says is true, and he indeed has the ability to save all humanity, the cost being the direct sacrifice of one million people... is just too insane!
To be fair, if this choice were presented to Wang Tao, he wouldn't be sure if he could make the call, considering it's the lives of a million people! Even if this really could save all of humanity, Wang Tao isn't a robot; he might not be able to remain absolutely rational...
"You're a madman..."