Circular reasoning is a fallacy in which the conclusion is used as an argument in its own defence. However, although circular reasoning is a fallacy, it is actually completely unavoidable. What do I mean by this? Simple: How do you know that human beings have the ability to reason?
With this one question, every discussion we will ever have could potentially fall apart. There is, after all, no point in attempting to reason if you are not capable of doing so. Thus, every conversation you ever have depends on the above question. But answering it always requires circular reasoning, because any attempt to prove our ability to reason must first assume we have the ability to reason in the first place. And if we lack the ability to reason, how do we know our reasons for believing we can reason are genuinely reasonable?
This is especially problematic when you consider the wide range of weird and wonderful beliefs in the world. There are various religions, various political ideologies, various theories about both the natural and the supernatural, there are even people that are considered insane. The very phrase "mental illness" requires a standard of mental health, and that this be met by those who identify mental illness. So how do we know we're not all crazy, delusional, maybe even just plain stupid?
And so there is a scenario in which circular reasoning ought not be a fallacy. Otherwise, we're all locked into an eternal loop when every statement we make is fallacious, even if it is, in fact, logically sound. And so the question is, what is that scenario? Enter Yahweh, the fear of whom is the beginning of wisdom. Christianity is the only major world religion that fully accounts for human reason without being a fallacy.
This is because Christianity teaches that God is omniscient, omnipotent, and designed us with the ability to reason, as well as the command to do so. Because He is omniscient, He not only knows the truth of every matter, but knows how to give us the ability to access some of that. Because He is omnipotent, He is fully capable of giving us that ability. He has also commanded us to use that ability with the specific intention that we get to know Him.
Thus, we have a solid starting point to our circle. We are no longer relying on a maybe, but a definite. To prove our ability to reason, we are no longer appealing to what is fallible even by our own reasoning, but instead to an infallible source. No other religion on the earth, at least as far as mainstream religions go, solve this dilemma. In fact, many of them bring us to the conclusion that we cannot, and should not, trust our reasoning abilities. Thus, to take any other mainstream religion seriously is to question our ability to take those religions seriously. Christianity wins just because it is the only religion that gives us reason to believe we can have this discussion in the first place.
God doesn't even expect us to leave it there. Not even Jesus, though being God Himself, expected blind faith. His works bore testimony to His authority (John 10:25; 37-38; John 14:10-11). Chief among those works? His own resurrection! No one gets up from the dead, but Jesus did, removing all reasonable doubt that He really is who He says He is. And so why not believe Him? His is the only religion that affirms our ability to reason, and the only religion with good reasons to believe. There is no sense in any other. The fear of the Lord is the beginning of wisdom, and belief in the Lord is the beginning of salvation.