King Theoden reigned over Rohan from the age of 32. He rememeberd his grandfather and was just five when his father became King of Rohan.
He honourably took under his wing, his two orphaned nephew and niece...
and yet this honorable, traditional, man, Lord of the Mark, born into tradition and honour... allowed himself to be overtaken by the cunning and wile of a wizard...
This man, who when released from Saruman... became one of the stronger campaigners in Gondor.... was also one of the weakest during the Third Age...
Do you not think that someone with such obvious history and background strength, should not hvae given in so easily to the wile of Saurman

He did not give in so easily to the deviousness or suggestion of any other .... (and it cant just be because Saruman was a wizard ..............)