What are the catholic letters in NT holy scripture?

Answer 1

If you mean catholic (with a lower-case "c") in the sense of "universal," the teachings of James, 1&2 Peter, 1, 2 & 3 John and Jude are applicable to Christians at large as opposed to a specific person or congregation (even though 2 and 3 John are addressed to individuals, their teachings broadly apply to all believers). As a group, these books are often referred to as the "general epistles."