Having proven the necesity, sufficiency, authority of Scripture, and then looking at the extent of inspiration of Scripture; proving the verbal, plenary inspiration of Scripture, then the inerrancy of Scripture and the Canon of Scripture, I would now go into the fundamental issue of the key concept of Sola Scriptura: that of ultimate authority.
As it can be seen, I have been using Scripture so far to prove all its own attributes. Of course, epistemologically, one must take into account one's own holy text in order to find out what it says about itself. Through the use of Scripture and a few other evidences, I have managed to prove all the above attributes of Scripture. However, isn't the whole argument so far just an exercise in circular reasoning? Using Scripture to prove the attributes of Scripture; using Scripture in fact to prove and undergird almost every argument for these attributes, seems like reasoning in one big circle. So why should our arguments to be taken seriously at all, since circular reasoning is normally one big logical fallacy?
This whole idea of engaging in circular reasoning, however, does not constitute a logical fallacy in this instance. It must be reminded that the issue here is about ultimate authority. What I have done so far, in appealing to Scripture over and over again, is to treat it as my ultimate authority. In the post on the inerrancy of Scripture, it was stated that opponents of verbal, plenary inspiration would say that Christians (or fundamentalists as they call us) turn the Bible into a 'paper pope'. In the technical sense of the term, these opponents are right. We are treating the Bible as our ultimate authority.
Now, then, an ultimate authority cannot appeal to anything but itself, since by doing so it would acknowledge another to be an authority higher than itself and it is therefore not the ultimate authority. Therefore, epistemologically, all reasoning on ultimate authority would by necessity be circular. Let us see how the liberals and other non-Christian wordlview are actually circular also.
The old-time liberals of the Higher Criticism group, when deconstructing the Bible, are actually postulating that their intellect, or rather the collective intellect of the liberals, are to function as the ultimate authority in all things. There is simply no reason whatsoever that can be given as to why they are so sure that all of them by their intellect could not be wrong. They may admit that they could individually be wrong, but collectively, they would deny that they could be wrong since they believe that they could correct each other's mistakes. However, false premises give rise to false conclusions. If everybody share the same false premises, then no matter how much thought is given to the problem, the best you can get are valid arguments which are unsound due to the false premise embraced. This is the fallacy of Rationalism and all who base their beliefs on their mind or the collective worldview in society (like for example the scientific evolutionary establishment). In fact, this describe the entire discipline of philosophy apart from God and all secular humanist thought.
As for non-Christian religions, it need not be said that they have their religious books or teachings which are similarly circular in reasoning (unless they are read through the lens of rationalist discourse, which would come under the fallacy of Rationalism). Islam for example have as its ultimate authority in the Koran. Try asking a devout Muslim why something is correct and they will say that the Koran (or the Hadiths) say it is correct. Try questioning their holy books and they would not be able to answer that question except to say that the Koran (or the Hadiths) say so.
Since that is the case, the liberals and rationalists among others who attack Christianity by saying that all such proofs for its inerrancy etc are circular are by themselves engaging in circular reasoning. They have no right whatsoever to call us to account for doing so since no one can do otherwise.
But this then opens up a Pandora's box. For then how can we decide which system is correct? Since there is no such thing as neutral ground, upon which can one make an informed decision regarding the correctness of any religious/ philosophical system, how then can one make a correct choice? What makes one system better than the other? Some try to validate postmodernism by claiming such epistemologically nihilism. However, if postmodernism is correct, then there is not one system that is better than the other, except that the postmodern system is better than all exclusive systems (which goes to show that postmodernism is self-defeating). How then can one decide between different systems?
I, of course, as a Christian, would claim the Scripture as ultimate authority and God the ultimate authority over the world. Many other religions and religo-philosophies claim ultimate authority status for their respective books and figures and ideas also. What then differentiates the true and the false one, or the good and the better one? Since Christianity is an exclusive religion (Jn. 14:7), the contrast is between true religion in Christianity and the other false religions and philosophies. How one could compare them is by comparing the consistency of each religious or religo-philosophical system, since truth is consistent with itself. Test its internal consistency and also check its consistency with the external world, as they are supposed to describe the external world. It is my contention that Christianity passes both tests, and none others do so, and thus taking Scripture as our ultimate authority is the supremely and only rational thing to do.