Ran this in a Mary deck during a 4P game and was pleasantly surprised at how useful it was. The real benefit from this was in enabling me to move to others' locations and pull an enemy off them fearlessly, even when I didn't have the actions to finish the enemy off that round. It was also incredibly useful for neutralizing hunters about to enter our location while we still had business there. Knowing their attack would fall flat gave us substantial latitude to focus on advancing the act rather than worrying about proactively engaging baddies.

This isn't a card for every build, but in the right one, it can be refreshingly liberating.

I just don't see a place for this card. Compared to Obfuscation:

1) It's slow (takes an action to play, and hence, is prophylactic).

2) Requires tokens to be in the bag (and requires you to seal them, foregoing their benefit).

3) Has the same resource cost; takes up the same arcane slot; has the same "Spell" trait.

4) ..and costs 2 XP instead of 0 XP.

In exchange for all that, Shield of Faith cancels any attack, not just AoO's; and works for any investigator at your location.

So among investigators who can take both (Leo, "Skids", Zoey, and Lola), you'd clearly only be interested in this if you're leaning heavily into protecting your teammates. And if that's the case...well, there are better cards for that. Dodge, Solemn Vow and Heroic Rescue are all fast and cost fewer resources (ZERO for SV and the upgraded Rescue), require no XP, and the latter two complement Leo's tanky nature. He can certainly afford to take the occasional hit on the chin without sinking some considerable setup into canceling attacks. Skids is a fighty Rogue, whereas Zoey is a roguish Fighter; between the two, I guess Zoey likes this a little better since she's more likely to be engaged with an enemy during the enemy phase. I'd still rather have her resources committed to dealing damage instead of playing defensively, though. And as far as Lola...well, beats the hell out of me. If you're running a Lola bless-heavy deck that can leave a defensive asset on the board just to cancel some enemy attacks--well, that's just crazy, amirite?

Now, a few words about the value of canceling Attacks of Opportunity versus any type of attack. It's been said more eloquently elsewhere, but if you're unintentionally engaged with a ready enemy during the enemy phase, your game is already heading south. characters operate by killing enemies during their turn, and if one makes it to the enemy phase, they have several contingencies to choose from: expendable allies that provide benefits from dying; counterattack events, items, or allies that kill the enemy during its own turn; or some other situational reason why you wouldn't mind taking a hit. Simply canceling the attack with a card like Shield might have some value, but it lacks the forward momentum of these other options, and is therefore the least advantageous.

On the other hand, taking an Attack of Opportunity can sometimes be a necessary evil that you incur as a cost of keeping your tempo in place. Maybe a monster spawned on you during the Mythos phase before you have a weapon out to deal with it, and you need to play that asset from your hand. Maybe you need to drag said enemy to a new location--one with a combat-ready teammate, or a clue that you'll pick up by killing the monster. Or maybe you're Trish, or you're running with Alice, and you like investigating with an enemy in your face. The point is, AoO's are incurred while you're doing something to advance the scenario, not just standing there and getting torn up. So clearly I believe Obfuscation is the superior card among those who can take it--and that's only if you're playing a specific build that will benefit from canceling attacks at all.

And how many attacks are you planning on needing to cancel, anyway? If it's 1 or 2, why bother? Use a cheaper, faster option like Dodge. And if it's 3, 4, or 5...well, Arkham Horror isn't a game that rewards a heavily defensive playstyle. Being a punching bag isn't going to win you any games.

Lastly...what about the guy whose picture is on the card? A who might be interested in protecting himself and his teammates, and who has the ready ability to add blesses to the bag? Even if this card was 0XP--or paid for with bonus starting experience--I'd still pass. Don't forget that you've spent cards and resources to put those tokens in the bag, only for them to wind up in purgatory. I greatly prefer Rite of Sanctification with Mateo, to cycle those tokens back to where they belong faster while getting some assets out for your team: weapons, spells...you know, the type of things to ensure everyone will be taking fewer attacks from enemies.

TL;DR: Leave it in the binder. 2 resources, 2 XP, an action, a card, an arcane slot, and a bunch of sealed tokens just isn't worth it to cancel a couple of attacks.