The expression knight in shining armour denotes a person regarded as a medieval knight in respect of his chivalrous spirit, especially towards women. In A Concise Dictionary of Phrase and Fable (1993), B. A. Phythian explained:

Despite its medieval feel, this is a twentieth-century phrase, first recorded in print in Victor Canning’s Whip Hand (1965). It originates in the general romantic conception, found in old tales, fairy-stories and Victorian poetry, of the noble knight wandering on horseback in search of good deeds such as rescuing damsels in distress.

However, contrary to what B. A. Phythian and the Oxford English Dictionary (2^nd edition, 1989) indicate, the English novelist Victor Canning (1911-86) was not, in The Whip Hand (1965), the first user of knight in shining armour in its figurative sense. The phrase was in fact already a cliché. For example, The Beverley Recorder (Yorkshire) of 15^th August 1903 published The second epistle of Madge, chapter 13 of a novel titled How’s That? An Anglo-Australian Story, in which one A. B. Cooper wrote:

My Dearest Clare,—Your last letter struck me as being rather sad. I suppose you didn’t intend your low spirits to appear, but I fancy your pen betrayed you. What is worrying you, my dear? Surely not an ‘affaire de cœur’? Who is the lucky swain? But no, it can’t be that, for you seemed so terribly and irrevocably heart-whole when you were in England, and yet some day—some day—there will ride into your life a knight in shining armour, who will gather you in his arms, all willingly, and together you will ride away into the land of dreams.

An article titled Mark of the Beast, published in the Evening Chronicle (Newcastle upon Tyne) of 7^th January 1942, contains the following:

A leading German newspaper, in reviewing the present position, declares that if Germany were to be defeated “it would mean the annihilation of Europe, and the end of all that life means.” If Russia were to defeat the Fuhrer’s hosts, adds the writer, the world would be at the mercy of “cold-blooded American and Soviet Jewish domination.”
Do the German people really swallow this sort of rubbish? It is greatly to be feared that they do, regarding Hitler as a knight in shining armour, defending all and sundry against incredible perils.

The image of knights in shining armours is ancient. In his 1533 translation of the history of Rome by the Roman historian Livy (59 BC-AD 17), the Scottish poet and translator John Bellenden (circa 1495-circa 1548) wrote:

Þai war iijc and vj knichtis in schynyng armoure.