The circular reasoning fallacy is the fallacy of assuming one's conclusion as a premise. Sometimes, circular reasoning is blatant, and of course, as Christians, we are quite often accused of it. It is circular reasoning to assert that the Bible is true on the basis of its own affirmation of that truth, for example. Such an argument is internally consistent, because if the Bible was true, it would be acceptable for it to affirm that. However, the reasoning is still invalid because a false book, such as the Qur'an, would also affirm its own truth.
Other times, the circular reasoning is not so obvious. This is particularly the case when premises are multiplied, creating a larger circle. It's easy to spot the circular reasoning in "the Bible is true because the Bible says it is true", but it's not as easy to spot it in, for example, "Christianity cannot be true because it promotes poor moral values". Because the conclusion is not explicitly assumed in the premise, it's easy to miss the circle here. However, hidden within the assertion that Christianity promotes poor moral values is the assumption that Christianity is not true.
Let's imagine Bill and Ben want to measure a flower. Bill takes out a ruler and says "this flower is 23cm tall". Ben takes out his ruler and says "actually, it's 27cm tall." So they compare their rulers, and sure enough, they are different. Bill claims Ben's ruler must be faulty, because it measures flowers wrong.
The problem in this scenario should be apparent. On the surface, "you have a faulty ruler because it measures flowers wrong" does not seem like circular reasoning. However, Bill's assumption that Ben's ruler measures flowers wrong is based on the assumption that his measures flowers accurately. Thus, Ben's must necessarily measure flowers wrong.
In much the same way, in order to claim Christianity promotes poor moral values, one must first assume that Christians have the "faulty ruler", so to speak. But is this not the very conclusion the evil Bible argument is designed to prove?
But this does actually lead us into a problem. Regardless of who has the faulty ruler, the simple fact is either Bill or Ben does have a faulty ruler. How do we determine who it is? There are actually "official" calibrated rulers you can obtain in order to compare your ruler to. But even this is actually still circular reasoning. When you compare your ruler to a calibrated ruler, you are assuming the calibrated ruler is correct.
Similar principles apply when verifying things like coins and bank notes, passports, and other legal documents. Verifying them requires a degree of circular reasoning. Even arguing that we are reasonable beings, capable of accurately assessing truth claims, requires us to assume we are able to accurately assess the truth claim that we are reasonable beings. This requires us to accept a difficult truth: Circular reasoning cannot be inherently fallacious, as reasoning in a circle is completely unavoidable at the deepest level.
Generally speaking, however, circular reasoning is fallacious. Perhaps the best way to expose it is to literally write the argument in circular form. An example of this can be seen in
an early Bible Brain article regarding the alleged authority of the Roman Catholic Church to interpret Scripture. As you can see, the Roman Catholic argument, reduced to a single run-on sentence, can be written as an unbreakable circle. Scripture being the word of God is affirmed both within Scripture, and within Roman Catholic tradition. Thus, by its nature, it must logically be more authoritative than any Church. To say otherwise is to say a Church is greater than God, yet it is His authority to which they appeal. "We are the Church Jesus founded", they claim.
But if Scripture is more authoritative than the Roman Catholic Church, the Roman Catholic Church should never be in conflict with Scripture. In reality, however, it very often is. Yet, when this is pointed out, Roman Catholic apologists assert that the Bible must be interpreted in accordance with the teachings of Rome, who alone have the authority to interpret the Bible. To defend this authority, however, they must either appeal to official Roman Catholic teachings, like those of the Council of Trent or First Vatican Council. In other words, Rome has authority because the Bible says so, and the Bible says so because Rome has authority. Lacking the assumption of Rome's authority, the Bible becomes like any other book.