It seems sensible to present all the evidence first, but if it's all TL;DR then feel free to scroll straight down to the Argument. I'm looking to find out if my logic is sound and my conclusions are accurate per the rules.
Exhibit the -3rd: Cleric spells
PHB, pg.32: Clerics do not acquire their spells from books or scrolls, nor do they prepare them through study. Instead, they meditate or pray for their spells, receiving them through their own strength of faith or as divine inspiration.
Exhibit the -2nd: Cleric domains
PHB, pg.32 (edited for brevity): When you have chosen an alignment and a deity for your cleric, choose two domains from among those given on Table 3–7 for the deity. While the clerics of a particular religion are united in their reverence for their deity, each cleric emphasizes different aspects of the deity’s interests. [...] Each domain gives your cleric access to a domain spell at each spell level he can cast, from 1st on up, as well as a granted power. Your cleric gets the granted powers of both the domains selected.
Exhibit the -1st: Domain feats
Complete Champion, pg.52: Domain feats are a new category of feats that signify a character’s dedication to a particular religious ideal or tenet.
Exhibit the zeroth: Clerics and domain feats
Complete Champion, pg.53: In addition, you can choose to give up access to a domain in exchange for the corresponding domain feat [...]
Exhibit the first: Knowledge Devotion, RAW.
Complete Champion, pg.60 (edited for brevity): Whenever you fight a creature, you can make a Knowledge check based on its type, as described on page 78 of the Player’s Handbook, provided that you have at least one rank in the appropriate Knowledge skill. You then receive an insight bonus on attack rolls and damage rolls against that creature type for the remainder of the combat. The amount of the bonus depends on your Knowledge check result [...]
Exhibit the second: Using Knowledge to identify monsters
PHB, pg.78: [...] In many cases, you can use this skill to identify monsters and their special powers or vulnerabilities. In general, the DC of such a check equals 10 + the monster’s HD. A successful check allows you to remember a bit of useful information about that monster. For every 5 points by which your check result exceeds the DC, you recall another piece of useful information.
Action: Usually none. In most cases, making a Knowledge check doesn’t take an action—you simply know the answer or you don’t.
Try Again: No. The check represents what you know, and thinking about a topic a second time doesn’t let you know something that you never learned in the first place.
Exhibit the last: conflicting points of view
GitP forum user Flickerdart: Knowledge Devotion has nothing to do with using Knowledge skills to identify a creature. Its effects are clearly spelled out in the feat.
1) Make a Knowledge check as appropriate for the creature's type. 2) Receive bonuses depending on the check result.
The feat makes absolutely no reference to the Knowledge rules to identify a creature [...]
GitP forum user Torvon: Knowledge devotion is not a knowledge check to learn something about the creatures. Otherwise the bonuses you receive would be transmittable to your allies somehow. Knowledge Devotion as something like divine inspiration: you get bonuses, but you don't actually LEARN something about the creature which you could communicate to your allies (that is why only you get the bonuses).
GitP forum user GoodbyeSoberDay: When you make a knowledge check for Knowledge Devotion, you're not "trying again." You're using the skill to do something completely different.
GitP forum user Curmudgeon: How does [the bonuses being transferable] follow from what the rules say? The Knowledge check may give the character some information; whether they communicate that information is up to them. But Knowledge Devotion provides benefits only to the character who has the feat, because that's what it says.
- Axiom: The writers of D&D sourcebooks observe the Cooperative Principle, in particular the maxim of quantity: it is to be assumed that what is written is neither more nor less than sufficient information.
- Premise: Bits of lore obtained from Knowledge checks to identify monsters are transferable between characters.
- Knowledge gained through the Knowledge Devotion feat is explicitly not divine inspiration.
- The Knowledge Devotion feat does not use a separate Knowledge skill check from the one used to identify a monster, nor does it constitute a different use of the Knowledge skill.
- What is true of knowledge gained through the feat is also true of the associated bonuses.
- From 2, 3 and 4, knowledge gained through the Knowledge Devotion feat is transferable between characters.
- From 5 and 6, bonuses gained through the feat are also transferable between characters.
Is this logic sound? Are these conclusions correct?
As this is a deductive argument, it seems sensible to briefly discuss the correctness of the premise: Curmudgeon notes it in passing, but of course it would be somewhat ridiculous (and arguably defeat the purpose of the Knowledge skill, if not the entire game experience) to suppose that what characters learn about the world can never be communicated to other characters.
As regards point 3: observe that clerics gain spells by (some flavor of) divine inspiration. The effect of gaining a domain is to grant the cleric additional spells and a granted power (the use of the word granted here implying some more direct form of divine intervention - something or someone must be granting the cleric their spells and powers, even some cosmic force for those clerics without deities). Further observe that to gain domain feats, including Knowledge Devotion, clerics explicitly give up domain access. Further still, observe that any character can take a domain feat, because it represents "a character’s dedication to a particular religious ideal or tenet." To wit: it is the character's personal qualities that make them eligible for (Knowledge) Devotion, not their connection to a particular deity-level entity. Therefore, it stands to reason that, contra Torvon, it is the character's religious devotion to knowledge as an end-in-itself that grants them the knowledge described in the feat, not the intercession of a particular deity-level entity.
As regards point 4, it should be clear that this knowledge is gained over and above the knowledge gained by an identical character, with identical skill ranks, rolling an identical check, but without the feat; the knowledge is different in degree, not in kind. Contra Flickerdart, the feat absolutely does make reference to the mechanics of the "standard" Knowledge check; moreover, it refers specifically to the rules for identifying a monster in the PHB. Observing the axiom, it should be clear that, contra GoodbyeSoberDay, if it was intended to be a new use for the same skill, it would have sufficed to simply call it a Knowledge check, in the same way that a bullrush calls for a Strength check that is clearly different from the Strength/Dexterity check called for by a trip attempt.
Furthermore, observe that the rules on pg. 78 of the PHB make absolutely no reference to the transferability of lore gained through Knowledge checks to identify monsters. Curmudgeon asserts that Knowledge Devotion bonuses are not transferable "because that's what it says", but in fact neither the PHB nor Complete Champion discuss transferability at all. If we follow Curmudgeon's logic, and read absence of evidence as evidence of absence, lore from a "standard" check would not be transferable either, "because that (nothing) is what it says". This is, as noted above, patently ridiculous. Consider a scenario in which, following GoodbyeSoberDay, either the character learns important lore about the monster's vulnerabilities, or learns how to hit and hurt it better, but never both. Leaving aside how the rules specify that you either know something or you don't, and therefore almost never roll a Knowledge check more than once, this is also clearly ridiculous.
Wherefore we must conclude that bonuses gained through Knowledge Devotion are transferable between characters. QED.
The above has been an argument from crunch. Below are some alternative justifications:
The argument from Rule 0: I'm the DM and it makes sense to me, therefore it is so.
The argument from Rule -1: It increases the power and agency of the characters, and the players are supposed to have fun and win, therefore it is so.
The argument from fluff: The cloistered cleric is the prototypical example of a character who knows the best ways to hit things, but whose value is not in their ability to hit things (they sacrifice BAB, Fort saves and HD for their knowledge). The obvious conclusion is that they were meant to share their knowledge of hitting things with others.
The argument from the intersection of fluff and crunch: Bonuses are a form of (applied) knowledge; knowledge is meant to be shared.
The argument from Foucault: Knowledge only becomes knowledge in the context of a power relation, which relations obtain between characters; knowledges are inherently plural, meaning that an unshared knowledge that does nothing to affect the relations between subjects cannot truly be called a knowledge.