Sola Scriptura - what is it?

You have to go by the definition that those who believe the doctrine go by. Here is the most common summary of that doctrine. “ The Bible - [or Scripture] - alone is the only infallible rule of faith and practice.” That statement does not rule out other, fallible - according to Reformed standards - rules of faith and practice such as creeds, catechisms, and the works of important theologians including Church Fathers. It is NOT "Bible only.” The problem that I finally had to face up to is this. I will try to state it simply and clearly, so please bear with me. The Bible is infallible. Catholics and Protestants agree on that point. How then can a fallible rule of faith be trusted to interpret that Bible? I don’t think it can. There must be an infallible way to interpret the infallible Word of God. It is the Holy Spirit who makes the Bible infallible, not the decisions of men. It is the Holy Spirit who guides the Church into all truth. The Holy Spirit is not confused or divided, so He must have some way to 1. inspire Scripture 2. Preserve Scripture 3. lead the Church into all Truth. What is that way? It is called Church Tradition.

The Protestant Reformation did not give us the Bible. Catholic Tradition gave us the Bible. Catholic Tradition preserves the Bible. Sola scriptura was a doctrine invented just 500 years ago. It is not found in the Bible itself, no matter how much Protestants say it is. Yes, Protestants also love Scripture, but in general are not familiar with how we got our Bibles. We all have the Traditions of the Catholic Church to thank for its very existence. Before the Bible came the Traditions. Think about it. Jesus founded the Church. The New Testament was finished sometime in the 1st Century, but after the founding of the Church.

The Councils of Hippo and Carthage were when the Catholic Church determined what books actually belong in the Bible. [1.] It was officially settled then, and then reaffirmed at the Council of Trent. [2.]

It was not until the Councils of Hippo and Carthage that the Catholic Church defined which books made it into the New Testament and which didn't. Probably the council fathers studied the (complete) Muratorian Fragment and other documents, including, of course, the books in question themselves, but it was not until these councils that the Church officially settled the issue.[3.]

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1.the councils of Hippo (393 A.D.) and Carthage (397, 419 A.D.)

2It has thought it proper, moreover, to insert in this decree a list of the sacred books, lest a doubt might arise in the mind of someone as to which are the books received by this council.[4]
They are the following:
Of the Old Testament, the five books of Moses, namely, Genesis, Exodus, Leviticus, Numbers, Deuteronomy; Josue, Judges, Ruth, the four books of Kings, two of Paralipomenon, the first and second of Esdras, the latter of which is called Nehemias, Tobias, Judith, Esther, Job, the Davidic Psalter of 150 Psalms, Proverbs, Ecclesiastes, the Canticle of Canticles, Wisdom, Ecclesiasticus, Isaias, Jeremias, with Baruch, Ezechiel, Daniel, the twelve minor Prophets, namely, Osee, Joel, Amos, Abdias, Jonas, Micheas, Nahum, Habacuc, Sophonias, Aggeus, Zacharias, Malachias; two books of Machabees, the first and second.
Of the New Testament, the four Gospels, according to Matthew, Mark, Luke and John; the Acts of the Apostles written by Luke the Evangelist; fourteen Epistles of Paul the Apostle, to the Romans, two to the Corinthians, to the Galatians, to the Ephesians, to the Philippians, to the Colossians, two to the Thessalonians, two to Timothy, to Titus, to Philemon, to the Hebrews; two of Peter the Apostle, three of John the Apostle, one of James the Apostle, one of Jude the Apostle, and the Apocalypse of John the Apostle.
If anyone does not accept as sacred and canonical the aforesaid books in their entirety and with all their parts, as they have been accustomed to be read in the Catholic Church and as they are contained in the old Latin Vulgate Edition, and knowingly and deliberately rejects the aforesaid traditions, let him be anathema.

THE COUNCIL OF TRENT

Session IV - Celebrated on the eighth day of April, 1546 under Pope Paul III Decree Concerning The Canonical Scriptures 3. Catholic Answers -  Was the canon of Scripture determined before the Church councils that decided it?