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 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. Gríma is Anglo-Saxon for “mask”, Gálmód for “lecherous”: Wormtongue is Gríma son of Gálmód: “lecherous mask” or “masked lechery”.

 

[Ithilethiel]

When Gandalf the White revealed himself and raised his staff, "...there was a flash as if lightening had cloven the roof. Then all was silent. Wormtongue sprawled on his face."

The power of Gandalf the White was unleashed. With Wormtongue stunned his power over Théoden began to lift. Hope was reborn. Théoden himself announces to Eowyn, "The time for fear is past."

I believe the more Théoden was in Gandalf's presence the more Théoden returned to his right senses. That is what made him so readily heed to Gandalf's suggestions.

"It is not so dark here...I feel as one re-awakened." (Théoden)

 

[Deleted member 12094]

I think you are quite right Ithilethiel. Alcuin's point had made me wonder before as well.

Let me add another element to this "unleashed power" of Gandalf.

As we can read in the UT, upon Gandalf's arrival in Middle-Earth Círdan the Shipwright gave to him his Ring Narya which he had got from Gil-galad:

last came one who seemed the least, less tall than the others, and in looks more aged, grey-haired and grey-clad, and leaning on a staff. But Círdan from their first meeting at the Grey Havens divined in him the greatest spirit and the wisest; and he welcomed him with reverence, and he gave to his keeping the Third Ring, Narya the Red.

‘For,’ said he, ‘great labours and perils lie before you, and lest your task prove too great and wearisome, take this Ring for your aid and comfort. It was entrusted to me only to keep secret, and here upon the West-shores it is idle; but I deem that in days ere long to come it should be in nobler hands than mine, that may wield it for the kindling of all hearts to courage.’ And the Grey Messenger took the Ring, and kept it ever secret.​

Therefore, "Warm and eager was his spirit (and it was enhanced by the ring Narya)".

According to "The complete Guide of ME" from Robert Foster, Narya was the Ring of Fire (which can explain Gandalf's particular skills with fire, like during his meeting with Théoden, or with fireworks) but it also had the power to strengthen hearts (although I do not know where the author had the latter from). If true, then the last aspect was certainly meaningful for healing the weakened mind of Théoden after years of poisonous advise from Wormtongue.