Objection #1: Your argument implies you need to be omniscient in order to know that the revelation in question is from one who is omniscient.
This objection is given by rationalists or those who pose as rationalists for the sake of argument. It tries to take what reasons I give for the need for an omniscient person and throw them back at me. In essence, it seems to deny that discursive knowledge is possible.
The problem with this objection is that usually no explanation is given as to why it can’t be the case that the communication from an omniscient person is a or the self-authenticating sufficient condition for knowledge (link). That is, there is no higher standard, but there doesn’t need to be any higher standard. This is not to suggest that the communication itself can’t be the subject of other tests for truth – I’ve argued for several necessary preconditions for knowledge (link) – but the point is that no first principle or axiom can or need be subject to external validation. Scripture is what is says it is, and we can know it for that reason. There are indeed tests for truth without which it could not be the case it is what it says it is, but these confirmatory evidences do not on that account become premises according to which we base our belief in its divine origin. For in order for Scripture to truly be the sufficient condition for knowledge, it must be the case that these criteria too are prescribed by Scripture.
Objection #2: If Scripture is self-authenticating, why isn’t there more agreement that it is God’s word? And what about other people who claim they have self-authenticating revelations from a different omniscient person?
These are usually the follow-up responses to Objection #1. Really, the first question is little different than the question posed so often by Roman Catholics: if Scripture is perspicuous, why don’t people agree? In both cases, the answer is the same: people’s misuse of a sufficient[ly perspicuous] source is no indication that the source is inherently faulty. The noetic effects of sin, for example, is provided by Scripture as an explanation for these occurrences. See also divine determinism in Romans 9:1-6ff. as another or further explanation.
And as for competing claims of divine revelation or whatnot, this is where the tests for truth or necessary conditions for knowledge alluded to above – not to mention internal critiques – are really useful. As mere necessary conditions, they may not be able to function as the basis upon which one can justifiably infer what worldview is true, but given a true worldview via a sufficient condition for knowledge, they can function as bases upon which one can demonstrate what is false.
Obviously, there are any number of competing worldviews to Christianity. Infinitely many possible worldviews, in fact. Not only would it be impossible for any Christian to justify any knowledge if he first had to reduce to absurdity all other worldviews first, it would be impossible for any individual to do so with respect to any worldview. This is just to say that one must begin with truth before he can refute error. This may seem clear enough, but when a person proceeds to fault me for having accepted a particular worldview even though I haven’t examined all others – as if such were even possible – you have to wonder if they understand the implications of their [self-refuting] statements.
Objection #3: If truth can be self-authenticating, then an omniscient communicator is unnecessary.
I actually haven’t encountered this objection, but it’s one I wanted to mention so that anyone who does will have some idea of how to respond. The idea is that an omniscient communicator is just an unnecessary middleman. If the proposition that "not all truths are related such that to know one thing entails omniscience" is said by the Christian to be self-authenticating as communicated by an omniscient God, why not just take the proposition itself as self-authenticating and leave off it’s having been revealed by God?
The problem with this is that the one who claims partial knowledge is in no position to know of himself that, say, the doctrine of internal relations is false. In other words, he is begging the question. It may indeed be the case that the doctrine of internal relations is false, but only one who knows all truths would be able to examine the propositions He knows and determine such without having to rely on anyone else. Only such a person could then tell this to others who can’t rely on themselves. Or, as I essentially point out in one of the above links, the proposition that "it is self-authenticating that not all truths are related such that to know one thing entails omniscience" is itself in need of scrutiny in the context of all [other] propositions known to be true. But this requires omniscience. If true – and it is – we have not cut out the middleman but rather established Him as an epistemic necessity for us.