Paul cites a saying from "one of the Cretans" and calls it true, using it to justify a "sharp" rebuke so people may be sound in the faith. The point is correction away from false teaching and unhealthy behavior. See Titus 1:12–13.
In Titus 1:12–14, Paul cites a harsh Cretan saying from a local “prophet” as an insider witness to common Cretan vices. He then endorses the basic point and turns it into a pastoral strategy: Titus must correct “them” firmly so that they become healthy in what they believe and live. The goal is reform, not insult. This unit sits inside Paul’s instructions to Titus about stabilizing church life on Crete by appointing leaders and resisting disruptive teachers (Titus 1:9–11). Just before this, Paul describes certain people as rebellious talkers who unsettle households and chase improper gain.
Paul uses an “insider” cultural witness (“one of them, a prophet of their own”) to describe a setting where deceit and undisciplined living are treated as normal (verse 12).
A careful answer keeps the immediate passage, the book's argument, and the whole-Bible storyline together, so the question is answered in context instead of as a detached idea.
Start with Titus 1:12-13, then read the surrounding section so the answer stays connected to the passage's flow.
12One of them, a prophet of their own, said, "Cretans are always liars, evil beasts, and idle gluttons."
13This testimony is true. For this cause, reprove them sharply, that they may be sound in the faith,