Sola Scriptura vs. Scripture & Tradition

Start with the verse itself, in context. Paul tells Timothy in the two verses right before 3:16-17 to "continue in what you have learned and firmly believed, knowing from whom you learned it" (2 Tim 3:14) — an appeal to the person (Paul) who taught him orally, not merely to a text. "Complete, equipped for every good work" describes Scripture's usefulness, not a claim that Scripture alone, apart from apostolic teaching, is a sufficient rule. Two verses later in the same letter's companion passage, Paul commands: "hold to the traditions you were taught, whether by word of mouth or by letter" (2 Thess 2:15) — explicitly putting oral and written teaching on equal footing.

The strongest single argument, and the one to remember first: the canon of Scripture itself — the list of which 27 books belong in the New Testament — is nowhere stated in the Bible. No verse gives you a table of contents. The canon was settled by the Church's authoritative judgment (regional councils in the 4th century, ratified more broadly later), which was itself an act of Tradition, not Scripture. If you trust your New Testament's contents, you are already trusting a decision made by the Church acting as teaching authority, prior to and outside of "Scripture alone." Sola scriptura is, in this sense, self-undermining: it depends on a judgment it says shouldn't be trusted.

On "the Church invents new doctrines": Catholic teaching is that public revelation closed with the apostolic age: "we now await no further new public revelation before the glorious manifestation of our Lord Jesus Christ" (Dei Verbum 4; CCC 66). In 1907 Lamentabili (no. 21) condemned the claim that revelation "was not completed with the Apostles". John is traditionally the last apostle to die (c. 100), but the Church teaches the close of the apostolic age, not a dogmatic date. A later dogma such as the Immaculate Conception (1854) or papal infallibility (1870) is claimed to be a definition of what the deposit of faith already held, not a new teaching. Vincent of Lérins gave the test around 434: there must be "real progress, not alteration of the faith", growing "in the same doctrine, in the same sense, and in the same meaning", the way a body grows while it "remains still the same" (Commonitory 23). Nicaea (325) defining the Son's divinity is the familiar example. Grant the fair follow-up: "How do I tell development from invention?" Vincent's test needs someone with authority to apply it, so the real disagreement is again who decides. Protestants rightly cite Matthew 28:20 ("teaching them to observe all things that I commanded you") and Galatians 1:8; Catholics agree with both, and the dispute is about what the apostles handed on. Jude 3 speaks of "the faith which was once for all delivered to the saints": both sides cite it, Protestants for the sufficiency of Scripture and Catholics for a faith handed on whole and kept, not added to. Private revelations such as Lourdes and Fatima can occur, but they add nothing to the deposit and Catholics are not required to believe them.

• “Tradition” in the New Testament. The Greek word paradosis ("handing over") is used both for human traditions Jesus criticizes (Mark 7) and for apostolic teaching Paul commends (2 Thess 2:15; 1 Cor 11:2). Some translations render the positive uses as "teaching" and the negative ones as "tradition," which hides that it is one word.
• “The pillar and buttress of the truth.” 1 Timothy 3:15 says this about the household of God, the Church, not about a book. Ask: if the Church is invisible, where would anyone go to find it?
• 2 Peter 1:20-21. "No prophecy of Scripture comes from someone's own interpretation" points to the apostles, who received the meaning by the Holy Spirit and passed it on through their successors (see 2 Timothy 2:2).