I have to admit, though, I was wrong to suggest lying about the Count planning to legitimise this bastard.
Let's recap our response so far.
Bearing in mind that we want to be completely honest about the way that duty and loyalty mean a lot to us, while omitting irrevelant details like shadowy alliances with a criminal underlord in Curbiston called The Rat, a campaign of whispers that is full of slanders and calumnies against our presumptive lord, and our petty rivalry with Sir Owen... let's try to scrape something together than fuses these principles of openness, honesty, and streamlined truthfulness into a single coherent response.
How's this:
"I am a knight, my lord Marshal. Risking my life is merely duty for me. The blood that we spilled on the field of Torchester together ought to convince you that I am found in the front rank when the cause is just and the battle hot."
"I'm not going to hold the late Count's mistakes against his offspring. Arthur deserves a chance to prove himself. However, though I did make amends with the count some time before his death, you're correct in that loyalty to his son isn't my only motivation. To put it bluntly, Owen may well have been responsible for the murder of the last Count, and a man like that is a danger to all his vassals. How can I feel safe when such rumours are afloat of my liege? He who would murder a Count, scorns to do right by his own vassals. Surely, none of us can rest easily if that is true. That is why I must protect Arthur, who has no fault on his soul but being fathered by a Foles in the rightful lands of the Foles."
"Yet I am but a knight, my lord Marshal, not an experienced leader of men. I do not have the means to press his claim myself, and I can give him precious little protection in the long run, without someone greater to take up his claim. If you have come with this intention, I offer up my lands, my men, and yes, my life to your disposal."