One of my players wishes to gain an Affinity via a mystery cult in game. The natural question that arose was "Do the experience points I have already put into Craft Stone get the Affinity benefit?"
For example, he already has the ability at 4 (50 xp). If he gains the affinity, does his ability increase to 5? Or does only the experience he earns after that point subject to the bonus?
While we're at it, any insight into this question?
https://forum.atlas-games.com/t/hermetic-calendar/2340/1
I would not retroactively apply the virtue. What passed previously shouldn't be altered for the new state that results from the initiation. Any new experience through study, though, would benefit. (Remember that adventure experience does not benefit from Affinity.)
Now if the character somehow had a curse lifted, or somesuch, that altered his mind, or memory-- and he had always had the virtue and the skill... then in that case I would permit the retroactive application.
-Ben.
I would also not alter gaines XP. Because I'm a meanie.
If you think about what an "affinity" is, it's a connection, a perfect fit (or better than average.) If that didn't fit before, then it was harder to learn before. Now that it fits better, it doesn't make the previous effort any more efficient or productive.
Otoh, if, somehow, one got "puissant" after the fact, that is an "Aha!" moment, and a relatively huge leap forward in comprehension. (Reminds me of the thread a while back debating the diff between those.)
My 2.
I'm going to swim against the flow here and say I think that the new Affinity would retroactively improve the affected Ability.
An Affinity is a natural talent in an Ability which grants an improved understanding of it.
Gaining an Affinity with an Ability one was already proficient in would, IMHO, produce an epiphany that throws previous experience into a new light. With reflection it would quite easy for a character to draw upon previous experiences to improve their current understanding of a topic.
After all each stage of comprehension is not discreet in and of itself, but instead builds upon previous understanding. That's to say that each level in an Ability doesn't represent a discreet grade of knowledge but instead bookmarks where a character is upon the continuum of the body of knowledge and skill the Ability represents.
If you have a natural understanding of an Ability, represented by the Affinity, it should inflate your knowledge irrespective of when it was developed.
I'm going to agree with the majority here. The affinity should not function retroactively.
This is the point of departure from my opinion and I suspect the opinion of the others here. The affinity virtue as presented in fifth edition is not in my opinion a natural talent that grants improved understanding. It is an aptitude that allows the character to grasp deeper understanding from the experiences information they come in contact with.
Really? My copy of ArM5 doesn't go into that sort of detail (hence my reverting to a dictionary definition for "affinity"). However it does say the following (emphasis mine):
In the case in question, an Affinity gained after character creation, I'd stick by my guns. It appears IMHO that the RAW agree with me. 8)
Sorry if I sound smug. 
You didn't have the affi9nity at character creation. The character creation is long past, if you are being introduced into a mystery in game. Hence, yoyu cannot re-apply6 the affinity to the XP put there at character creation.
While you first put down the XP, you did not have such an affinity, so you wetre getting less from your experience in that area. Now that you have an affinity, you are likely to understand better what you are experimenting, so you will get a better payout for your XP. Easy 
But YMMV. This would not be kosher IMS, but it might be in yours.
Cheers,
Xavi
Okay lets put it another way.
If you're creating a character who is supposed to have been initiated into a MC prior to beginning play. Now due to their backstory they're not initiated into said MC before a certain age but its evident that prior to their initiation they used an Ability which the MC grants an Affinity with during initiation.
Do you apply the Affinity bonus to all the XP spent on the Ability or only for the XP supposedly spent after initiation?
Keep in mind that ArM5 is supposed to permit characters created at a certain age and characters created younger and played up to that age to be roughly equal in ability.
Either the rules work equally during all stages of character development or they don't. I'm going with the former rather than the latter, otherwise I'd have to rationalize the transition between "creation" and "play".
But as you said, YMMV.
According to the original poster....
In game, not at charaxcter creation 
Quite a difference here. Basically, ther is no "catching back". From the point that you have been initiated in the affinity onwards, yoyu have an affinity, but your previous experience points do not change as a consequence of that.
PC development is way more detailed than "rough mage" development that is used for NPCs of a certain age.
Xavi
I don't believe that this level of snark is warranted by my post.
Here's an argument for my opinion.
Your copy of the book does in fact go "into that sort of detail". It says, as you quoted, that when the character hzs an opportunity to learn they gain more experience points for it than someone who doesn't have an affinity. If it were a deeper understanding then the character with an affinity could derive more from a summa than a character without the affinityl. But this isn't the case. The character gets the same understanding. they just get it faster.
The presumption of the rules is that the character had the affinity all of the way through character creation.
honestly, I'd have the xp and initiation tracked out and award partial, but I'm ridiculous-detailed oriented like that.
-Ben.
Says who? (My copy of ArM5 doesn't go into that sort of detail...)
That bolstered by the fact that the rules for gaining Affinities later, after CharGen, didn't exist when the core rules were written.
Now that they do, they may work the same, or not.
Planet, we hear what you're saying - you've said it three times now. Most just don't seem to agree.
No snarks were intended. If any appeared then I apologise. (I'm just glad it wasn't a boojum; sorry I couldn't resist.)
Three times? It must be true then (To a given value of truth). Okay I'll shut up...
Oh and please call me Niles, Planet is just my waistline. 
Personally, I don't think it should increase xp already used.
However. That means the affinity will probably only give the character a +1 increase to his score, and take a while to happen.
Whereas retroactively increasing xp could give an immediate +1 increase, and a further +1 over time.
The latter seems a better reward for undergoing initiation. So I don't think it should be retroactive, but i'd do it that way anyway. Seems fairer, and more fun for the player (and an extra +1 on an ability is not going to break your saga).
Well... sorta. I am a player in Rex Minimus's game (but am not the player in question). We are playing through our apprenticeships. "Character Creation" was just choosing our childhood skills. We all started at age 5 when we woke up in a house together and none of us spoke each other's language.
I don't know if this changes anything for anybody, but I thought it should be said. The PCs are 8-10 years into apprenticeship... not yet full blown magi.