Malory’s version is not original. Sir Tristram de Lione, one of the episodes of which a medieval chivalry literary piece, La Morte D’Arthur consist, becomes the most famous because of its adaptations as Opera, for instance. Each episode is a sort of “Orature”, as it were, a story which one has passed on to others orally. And All stories were gathered and compiled into one volume, La Morte D’Arthur. It’s impossible here to mention the complicated background of editorial process, but it is likely to be a political or religious purpose to have had a strong influence on the establishment of the book. (There are several variants in other countries.)
If you read it to understand what chivalry is, you might be puzzled so much. That is why it is interesting. Launcelot, allegiant to King Arthur, betrays him to steal his wife, Tristram and Isoud get close using the aphrodisiac. Everybody does what they want in spite of themselves. Therefore we strongly doubt the spirit of chivalry—a stereotypical figure like a gentleman, but stout versatile and considerate.
As to the spirit as such, that it is associated with Christian faith is significant. Tristram shows mercy towards the Queen who tried to assassinate him, or decides to do battle for the sake of his country at the risk of his life, that is to say, he is the embodiment of self-sacrifice and benevolence. Considering the respects, it is not unnatural that the Arthur’s myth was used as propaganda and in order to achieve the religious purpose. Since it is an oral tradition so that it is not created by a author as a coherent tale, but it is remarkable historical fact that such tales are compiled into a book after hundreds years passing.