Why did Théoden suddenly trust Gandalf?

[Hogarth]

Originally Posted by Varnafindë

I agree with Valandil - Theoden's eyes were opened. Gandalf set him free from the evil influence that Wormtongue had had on him.

Gandalf's staff may have been used to display or even transfer his power and authority - and it's also reasonable to think that Narya had something to do with it. But to set Theoden free to be persuaded by Gandalf, not to compel him to obey him.

Yeah, I agree. Gandalf dispelled whatever charm Wormtongue had set upon Theoden and Theoden regained his former senses. IT wasn't so much that Gandalf influenced Theoden to do something he didn't want to do, he just helped him remember his past glories and forget about the weakening influence of Wormtongue.