That's the problem, the way you're presenting it doesn't mean anything. Aside from this issue in your circular argument where you don't define what the right thing is, or who has the authority to pronounce something as such, we go back to the main issue that this is a game.
You haven't -asked- what the right thing is. Should I take this as changing that to "you hadn't"?
By the way, its something like this:
To fear God and maintain His Church
To serve the liege lord in valour and faith
To protect the weak and defenceless
To give succour to widows and orphans
To refrain from the wanton giving of offence
To live by honour and for glory
To despise pecuniary reward
To fight for the welfare of all
To obey those placed in authority
To guard the honour of fellow knights
To eschew unfairness, meanness and deceit
To keep faith
At all times to speak the truth
To persevere to the end in any enterprise begun
To respect the honour of women
Never to refuse a challenge from an equal
Never to turn the back upon a foe.
No, you don't have to perfectly uphold every one of these.
Not many people are going to get too upset about not doing right according to the arbitrary and ill-defined standards of a bunch of imaginary people controlled by their GM, and that is not some failing on their part.
If there is no measure in the game to track how good or evil you're being, why would anyone waste time thinking about it? You used that argument yourself some time ago, why is it different in this case?
Then they should not play characters who are supposedly doing the right thing by whatever standards. (Which is not the same as "playing this game", I will note. If I have to repeat myself, I'm going to laugh at the persons.)
As for why is it different? Because while there is no mechanical "100 Goodness points", there is an actual definition with an actual meaning to live up to and observable effects of doing so - just because you don't profit doesn't mean nothing happens.
Also, you have to allow for the idea, if this game is meant to have any replay value at all, that people can make different choices. There doesn't have to be a 1:1 relationship between good deeds and reward, but there periodically has to be something to reinforce that behavior.
The only thing there has to be is the fact that if doing good deeds makes the world a better place, there is that. If you want a "better" reward than that, I make no promises.
Also, can a player perform the proper deeds in your game simply through intellectual knowledge of what you or the game wants as right? Or would you somehow penalize that player for faking it? Is it better to intend good or do good?
(In setting) knowing what is Good is not particularly hard. Being able to actually uphold that is - and I do intend to make that part of the game (as distinct from just the roleplaying).
Trust in the Emperor, but always check your ammunition.