Not being affiliated with a church doesn't mean you don't know anything about religion. In this case, your Knowledge (religion) checks would solely represent the more common knowledge you have of Undead and Mythic History. Really, this is a good thing. It's something you want regardless of whether you're attached to a church. :)

Ultimately it's up to your DM ... he could choose to add Knowledge (Undead) as a new subcategory of Knowledge. But you'd be selling yourself short to take it. Go ahead and take Know (religion). It doesn't mean you're part of a church. Just that you know lots of Clericky Religious things. ^_^

I agree with damnedmage.

However, being as how you're not affiliated with a particular sect or deity, it would seem that Knowledge (Religion) would be applied to your specific philosophy (a Cleric has to get their power from somewhere, after all). This being the case, and considering your focus on undead-slaying, I would rule that undead expertise falls under your specific Knowledge (Religion) checks.

The core D&D rule for adjusting one's class is "ask your GM."

But before you do that, take a look at the 3.5 knowledge skill:

http://systemreferencedocuments.org/35/sovelior_sage/skillsInt.html

Knowledge (religion) *Does* cover knowledge about the undead. Chalk it up to understanding the passage of souls if the simple "it makes all knowledge skills equally useful" answer doesn't satisfy.

I believe D20 Modern has some discussion on making up new knowledge skills: suggesting that the more you invent, the more useless each one becomes and the weaker knowledge-focussed characters become as a result, so it is best not to, even though (in that game) every character who knows "Science" is like Marshal from "Alias" i.e skilled in any science with no speciality (at least not one that has any effect on game mechanics).

If the DM knows your background they can always give you a circumstance modifier on a given roll based on how much to do with undead the Religion roll is, or you can just roleplay "feeling very lucky" each time you succeed on a roll that is not to do with undead.

I agree with the others that just because you're not the traditional deity-focused cleric doesn't mean you don't know a lot about religion in general. It could be that you've learned quite a bit about religion and you simply haven't found what you consider worthwhile within the/a church. Or decide as a player to intentionally not use Knowledge (religion) for the more traditional stuff to represent the fact that the character either doesn't care about that stuff or maybe doesn't know it. Just because you have a skill doesn't mean you have to use it.

Also, don't forget the rules state "If you have 5 or more ranks in Knowledge (religion), you get a +2 bonus on turning checks against undead." I could see a DM allowing a similar bonus for Knowledge (undead), if the DM allows the skill in the first place.

Personally, I'd allow Knowledge (undead) to be a skill, and a class skill at that (maybe replacing Knowledge (history)), but I'd warn the player that its use is much more limiting.

To make it a clean sweep...keep Knowledge (Religion) due to it's ties with things non-church related that you will need to make checks for.

If you have use of Complete Divine, or any other current supplement book, look at some feats that let you use your turn undead attempts for other things for when you aren't going against undead.

...unless you're playing in a heavy undead world that is...

What has been said is worthwhile, but I also suggest that the character have some source of information. Check with the DM about education in their campaign: who can know what. If it's pretty medieval, with knowledge only being passed down familial lines or through apprenticeship, then the character isn't likely to know anything unless they had a willing teacher and perhaps access to this teacher's library.

In more serious campaigns, a temple would only provide education to those who served as priests, clerics, or paladins: someone who took up a sacred vow to serve. This does not mean you wouldn't be able to learn from a bard or other educated person.

Being an undead slayer is actually quite a commitment, as is being a cleric: clerics differentiate from normal priests as they are muchlike the sacred fighting orders of medieval europe, in origianl concept and they don't resemble the average priest of any faith, even a fantasy faith. Also, the Player's Handbook describes clerics as being members of temples, always.

A person doesn't just pick up a weapon, put on armour, and decide to serve a deity: the character class as described in the game rules is the result of long training. Remember, it is suggested in the DM's Guide that it takes about a week to learn a skillpoint and two weeks to learn a feat. I'd guess it takes about 28 weeks, almost two hundred days, to become a cleric, all considered: their education and devotive training, their weapons and armour feats, and other considerations. To be considered for clerichood might even take longer, proving oneself as an acolyte -- note the starting age listings in the Player's Handbook: from a minimum of two years for a half-orc or human cleric, a maximum of sixty years for an elf.

Most priests would be commoners, adepts, or experts, depending on the temple and their duties and training.

Oh, I just realised that quite a bit of clerical time might be taken praying and observing religious rite during their training: this might double their training time, assuming half their time is spent in such particulars, so that it would take around 56 weeks: about a year.

A normal assumption is that a person usually spends a season or two as a novitiate, then a year as an acolyte, then a bit more than a year training to be a cleric. Longer periods between acolyte and cleric might be spent during periods without extensive war, because the acolyte would be expected to take up mantle as a formal priest with priestly duties: at any rate, in most faiths, a cleric would be granted priesthood and all the benefits and obligations pertaining... although a cleric would be, in essence, a sacred soldier of their faith -- not a holy knight like a paladin, but a member of a military religious order of their faith.

...like everybody said, it's technically the same thing. You don't have to make knowledge checks if you don't want to make them for any given thing, it's really just that simple.

A rose by any other name. :)

In all my campaigns, the clerics MUST serve some sort of deity. The power to turn undead needs a connection to the positive material plane that is provided by ones deity. The power to cast spells comes from the deity. This is a blessing and a curse because a deity can choose NOT to give you a particular spell based on its philosophy. For example: A god of healing would NEVER grant a cause wounds spell. This also falls into the range of domain spells also. Without a deity, you could not choose a domain. Therefore, you cut down on the extra spells and bonus powers granted by that domain.

As always: Ask your DM. They have the final say.

My $ .02

Don

Knowledge (religion) covers more than just the ins and outs of the church. It covers the cosmology of whatever campaign in which you play. This also means that it covers undead and their workings as well as various aspects of divinity and the actions and aspects of souls, whether or not it's connected to a god or goddess. Undead are an aspect of the soul, as planesdragon pointed out nicely.

Also, the rules say that undead are covered by the religious knowledge. ;)