This card is probably going to allow 'heal decks' to become a thing.
Right now, there are cards that ask gently to have a 'heal engine.' Cards like Field Agent, Beat Cop (2), Agency Backup, Guard Dog, and Spirit of Humanity allow you to convert healing into positive tempo.
The problem with these cards is that while the combination is positive tempo (You can trade, for example, a card and action from Soothing Melody into 2 clues or 2 damage with the 'pain for gain' guardian allies) you need to hit very exact numbers to actually get those benefits. If you heal 1 damage from beat cop but he doesn't have any lost sanity (or to be honest even if he does a bit) you are paying a card and action for just 1 damage, which isn't great. It is... fiddly.
Soul Sanctification makes focusing on healing allies a lot less fiddly because you now can utilize healing cards to get a fairly good effect whenever you have excessive 'flex-heal' or when you have a heal and nothing to use it on. +2 to a skill test for every point healed often is going to be just a bit less efficient than these little auto pings of damage and clues. You are paying a lot for the ability to just ignore healing the asset (and you need to not heal the asset for 'single target' heals), but luckily it is also good enough to run as its own self contained strategy, which is what I think really seals this as an 'archetype.'
Good heal cards in Arkham tend to heal at least 2 per-action, meaning that Soul Sanctification allows them to become a more flexible activation of Encyclopedia you can bank. And that is a lower end good heal card, you can do MUCH better.
For example, in survivor and mystic, assuming you can hit at least a 6 test value on Earthly Serenity, your looking at 'spend 2 actions, get +12 spread across important tests. If your Daisy, you can utilize medical texts alongside or instead of Dream Diary in order to constantly generate 2 phantom unexpected courage cards every turn which is an extremely competitive use of her free book activation.
Not every deck wants to run this, but it completely changes the value of healing for a lot of characters. Outside of the obvious of Carolyn and Vincent, some other good candidates for this card include Mark, Leo, Daisy, and Skids (especially || front, X back Skids) of all people.
Edit to clarify how strong the impact of this card is, because I nerded out about the conceptual space it opened and burried the lead.
Its easy to think 'its just a skill test bonus' its critical to look at how strong any sort of spammable skill test bonus is in Arkham. Cards like Well Connected and Dream Diary (which Soul Sanctification compares to extremely well) are super valuable because guaranteeing a pass on anything but autofail once per turn tends to be a game winning effect, and more impactful than being able to do something like guarantee an extra clue once per turn, because the main way you lose in Arkham is getting put behind on tempo by negative effects from the scenario. Clue acceleration helps you catch up, but often the best strategy is to just literally never fall behind.
This may be the best card of that genre now. While Well Connected provides massive over-success numbers, it costs you something to fire it twice per turn. While that fits into the decks it wants to be in very well, it limits its generic applicability in some ways. Dream Diary, if you are dreaming the 'correct' dream, is usually an 'auto-sans-tentacles' once per turn for the mere cost of a hand slot, but you can't bank it, meaning that while it applies to the most critical test of your turn, you can't 'burst' with it in the same way you can as well connected.
What makes soul sanctification absurd is that for every 2 healing you can generate, you get a better version of essence of the dream that will stack until you use it, and even 1 healing will match it depending on context. This is a huge deal because it means you can do things like run your 'essence of the heal' for mythos defense without reducing your ability to use it to land a clutch attack, or to solve an important scenario test, or do whatever. The only cost is how hard it is to generate healing.
The worst case scenario is usually that it becomes a very good upgrade on encyclopedia to do whatever your doing: wasting 1 action to generate +2 on two different actions. That is still a very good use of a card and some XP, encyclopedia isn't bad (even outside of Daisy) and the flexibility of converting dead actions into future value is huge. Imagine if every action you didn't take let you pass a mythos card test automatically. That would be an action worth taking most turns on characters with middling mythos defense, and even really defensively solid characters would take it often.
But it can get SO much better. If you can find a way to either get more than 2 healing off a card, or to get healing without spending actions, suddenly you will start generating a LOT of offerings for soul sanctification that start pushing you to the 'never fail tests' category of play, which can be silly.
Its sorta like taking the time to generate 20 resources for a well connected deck. Even though generating 20+ resources isn't a normal goal for decks, the promise of never failing a mythos test and generally passing any critical test on your turn on top is so potent it is worth not just doing that, but making significant sacrifices to do that.
When you make that comparison, soul sanctification is clearly 'easier' to get test value from: you get a +4 for generating 2 healing on 1 action, which is going to be easier and often more repeatable for a deck than generating 8 resources over even 2 actions, but its also more 'bursty.' Well connected takes a lot of effort to 'take off' and then basically rockets to the moon, while soul sanctification 'coasts' more: You never are going to be without it when you really need it and you have a WAY stronger level 0 and early scenario deck than well connected decks, but your going to probably 'smear' the actions you sacrifice to soul sanctification over a scenario more.