... that is a specifically important bit of information that the Knights would need to know to keep them from trying to save somebody that is beyond help OR condemning somebody that isnt. The means and/or possibility of removing a Denarian's influence from a host is just plain too fundamental to their purpose and Power to risk them not knowing everything the Church does.
The Church has a method, IIRC: forswear all worldly power & action, live a life (with Church support) of penitential prayer & meditation. Eventually, the Enshadowed can become cured of the Shadow. I believe Michael tells Harry this. It's the method Michael knows, from Church teachings; this means that the Knights should try to turn the Enshadowed over to the Church hierarchy, rather than holding them and trying to talk them 'round "in the field" (I mean, I'm sure they DO try to persuade the Enshadowed for as long as they are with them... ) . Church doctrine seems to be that the Enshadowed CAN be saved (via seclusion).
The Church *MAY* (we have no evidence for this, either way) have some other method(s) that they regard as non-usable "in the field." They wouldn't necessarily reveal these to the Knights. If the Church has NEVER seen a mortal
independently resist a Shadow (and by Lash's testimony, they wouldn't have), Michael was showing extraordinary faith in Harry!