Circular logic is a fallacy that goes like this:
1. Claim A is made
2. Evidence for claim A is asked for
3. Claim A is said evidence
Since most sola scriptura Christians believe the Bible along is sufficient, they use circular logic to justify sola scriptura. Like this:
1. The Bible is God's word
2. Why?
3. Because it says so
The Bible is God's Word but the Bible does not suffice as the authority to say it is. Some sola scriptura Christians say Jesus is the sufficient authority, yet, this is not convincing if they say the Bible only testifies to Him: It's, once again, circular logic.
1. The Bible is God's Word
2. Why?
3. Jesus says so
4. Why?
5. The Bible says so
Or, replace 5 with "The Bible says He is God" or "Jesus says He is God", it's all the same fallacy.
What is needed in order to upload the Bible - for want of a better phrase - is something older than the Bible that comes from Christ, for, as Christians we hold Christ as our Lord and Savior, so we believe Him and follow His teachings. In addition, that something must be identical to what the Bible records about Christ and what the Apostles teach in the Bible, for, the Bible is the inerrant Scriptures and God would not teach something later which He did not teach earlier (in order words, the Bible must be identical to that from which it had come, otherwise we may suspect of error).
That something which the Bible comes from and which comes from Christ is called Tradition. It is the oral tradition of the Gospel, the passing down of the life and teachings of Christ by word of mouth, which God preserves from error and which He gives a deeper understanding to throughout the years. Without this Tradition, we could not claim, no matter how old the Bible is or how early to Jesus' death it was written, that the Bible is true, because, as I said, it would be circular logic to claim the Bible is true based on the Bible.
Tradition is based on the Jewish understanding of the Word of God. In Judaism, the Word is the Torah and the Oral Torah from which the Torah came: I.e., Scripture and Tradition. The Word of God is not, as most Christians believe, simply the Bible. That, in fact, would be scientifically impossible: Because, as archeology shows, oral tradition proceeded written writing in human history.
But, how do we know that Tradition is true? Wouldn't it be circular logic to say Tradition is true based on Tradition? Yet Tradition comes from Jesus, it is the life He led and the words He spoke which His eyewitnesses - the Twelve - witnessed to by their lives and teachings, so, Tradition is based on Christ. Yet how do we know Christ is God? And how do we know He existed? We have relics of Him, so there is historical evidence of His existence, and we have Roman accounts of His miracles, so there is historical evidence of His work, and we have His words, for He said, if one does not believe in His words to at least believe in His works, for they are the works of God. And from there we get into questions of God's existence, essence, and nature.
But to get back on topic: Sola scriptura is based on circular logic. Ironically, some sola scriptura Christans hold to their own versions of Tradition, saying that the Tradition of the Church is false. Again ironically, they based their own traditions on Tradition, just as the based their own idea of "church" on the Church. This would all be false if the Church had no proceeded the many churches which came from the Reformation, and these same churches came up with their own ideas on the Word of God and Christianity, yet, again as archeology shows, the Church did proceed the Reformation, and in fact, the Reformers were Catholics - Catholics who committed the sin of schism, that is.
While we all believe in Christ and follow His teachings, we do not all follow Him correctly. Only those with the fullness of the truth follow Him correctly, because, having the full knowledge of Jesus, they know Him fully, and so, fully adhere to Him, whereas everyone else simply adheres to Him according to what truths they possess, such as the truth of the Word being Scripture and Tradition. Now not everyone lives a Christian life, yet sin does not subvert objective truths - any more than a criminal subverts justice.