Tradition
2 Peter 1:20-21
Understanding this first, that no prophecy of scripture is made by private interpretation. For prophecy came not by the will of man at any time: but the holy men of God spoke, inspired by the Holy Ghost.
Context of the passage
Peter defends the reliability of the prophetic word against those who doubted the apostolic witness, grounding Scripture's origin in divine inspiration.
Catholic reading
If Scripture is not a matter of 'private interpretation,' an authoritative, Spirit-guided interpretive body—the Church's Magisterium—is needed to interpret it correctly, rather than individuals reading it in isolation.
Common objection
This verse is about the prophets' own origin of their message from God, not about how later readers should interpret Scripture.
Response
Even granting that reading, the same principle applies analogously: if the Spirit who inspired Scripture did not leave interpretation to human whim then, he would not abandon the Church to private whim now, hence Christ's promise to guide the Church 'into all truth' (John 16:13).