Why did Théoden suddenly trust Gandalf?

[Alcuin]

In Two Towers, “King of the Golden Hall”, Théoden greets Gandalf gruffly, calling him “Stormcrow”:

[W]hen I heard that Shadowfax had come back riderless, I rejoiced at the return of the horse, but still more at the lack of the rider; and when Éomer brought the tidings that you had gone at last to your long home, I did not mourn.

Wormtongue then adds his derision, insulting Galadriel almost as an afterthought, calling her “the Sorceress of the Golden Wood,” saying that “webs of deceit were ever woven in Dwimordene.”

Gandalf responds with what seems to be an old rhyme of Rohan concerning Lórien and Galadriel. Then he casts off his old, grey cloak, revealing himself as a shining figure in the darkness. (A storm is passing over Meduseld at the same time: they soon see it moving westward when Théoden steps outside.) He declares, “I have not passed through fire and death to bandy crooked words with a serving-man till the lightning falls.” And it seems lightning does fall – from Gandalf (though he normally seems to use fire rather than lightning) or from the passing storm from the East – upon Wormtongue, who “sprawled on his face.”

Immediately Théoden rises at Gandalf’s invitation and steps outside.

Why does Théoden’s do this? Gandalf just struck down his counselor, and whatever the rest of the court at Meduseld thought of Wormtongue, Théoden trusted him. Is he going with Gandalf out of fear? is it awe (related to fear, though different) because of the display? has Wormtongue’s mask* been torn partly away so that Théoden can see him in part as others do?

Why does Théoden suddenly rise and go outside with Gandalf, listen to him on the steps of Meduseld, and in a trice change his whole demeanor, intention, and direction?

─╫─

* I mean this as a symbol, of course: Wormtongue wasn’t wearing a physical “mask” separate from his face, nor did Gandalf injure the flesh of his face; but he presented to Théoden a “false face”, masking his true intentions. GrÃ*ma is Anglo-Saxon for “mask”, Gálmód for “lecherous”: Wormtongue is GrÃ*ma son of Gálmód, “lecherous mask”.

[Olmer]

There is a simple explanation to this strange action of Théoden, as it seems to me.
The power of persuasion to inspire others to do what the wielder of Narya wishes them to do, even without saying it. Théoden had no choice, but just to follow Gandalf's order.

[Valandil]

I have a more traditional - or a more "tame" - take on it.

I think Theoden's "eyes were opened" so to speak. He realized quite quickly the road he had gone down, and the way he needed to get back.

Alcuin - you've caught up to me, I suppose. I'm in "Helm's Deep" right now.

[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.

[Olmer]

Originally Posted by Valandil

He realized quite quickly the road he had gone down, and the way he needed to get back.

Quickly indeed.
Picture this: an unwelcomed guest came in your house . He is demanding, threatening, meddling with everything you are doing, and literally getting so much on your nerves, that in a desperation to get rid of him you are offering him your prized car. Which he is taking and disappearing for good, as you thought. But a few months later (Surprise! Surprise!) he is coming back, this time more bold and bully. You already was fed up with his contrivances at the last time, so he is definitely not welcome. You did not want to deal with him and asked your trusted friend to tell him to hit the road. Instead he is giving to your friend such a blow, so he is turning from a wise man into a blabbing idiot. Then he comes to you and speaks in a "low and secret" voice, so none save you can hear what he said. Probably making an offer you cannot refuse. And then you have a dilemma: to kick his butt out, or to follow the offer. You don't want to wake up with a horse head in your bed, so you are getting up and do whatever he is telling you to do.
Only aforesaid scenario explains such a mercurial "eyes opening."
Or we would rather agree on the supernatural power of persuasion?

[Butterbeer]

I just think it's a shame Marlon Brando didn't play Gandalf... if only that pesky ring had stayed sleeping with the fishes

Brando whispers to Theoden:

"Saruman's a pimp. He never coulda outfought Saurontino. But I didn't know until this day, that it was Barzini all along"

[Olmer]

Saurontino... he-he. On the other hand, we can approach the situation from a different angle.

and when Éomer brought the tidings that you had gone at last to your long home, I did not mourn.

Teoden, indeed, knew, or had a good guess of who really is this seems to be old conjurer, that he is not of this earth, his home is somewhere in the extended space, and therefore he is dangerous to cross with.
I'll take it that the wise Grima, who was unwillingly fell under Saruman's spell, had a pretty clear idea, who those wizards are, and what they can do to you, and shared this knowledge with Théoden.
So, after a quite startling demonstration of Gandalf's abilities, Theoden decided that it would be better to comply with this arrogant "son of a staff".

How does it sound? Better?

[Valandil]

So Gandalf "made him an offer he couldn't refuse"?

[Olmer]

Indeed, it was the offer, entirely irrational and totally destructive.
But, nevertheless, seems Theoden couldn't refuse. Otherwise how can be explained such a thoughtless action of the weathered in the wars king, who in such a rush, with so little logistic preparations, left the fortified capital, full of women and children, under protection of Eowyn and took his whole army in a hurry to fight Saruman , just because Gandalf told him so. Gandalf, who was up to that point entirely untrustworthy.

Yes, that was a very persuasive offer.

[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.