Titus calls the message he has just summarized a "trustworthy saying" and says it should be insisted on so those who have believed may be careful to devote themselves to good works. He adds that these things are excellent and profitable for people. See Titus 3:8.
In Titus 3:8, the writer points to a “trustworthy saying” and tells Titus to insist on these points with confidence. The purpose is practical: people who have come to trust God should take care to devote themselves to doing good. This line closes a larger movement in Titus 3 where the writer shifts from instructions about public conduct (3:1–2) to a reminder of God’s kindness and the new pattern of life expected from it (3:3–7).
Verse 8 treats what has just been said as a dependable summary: “this saying is trustworthy.” The writer then links that reliable message to a teaching goal: Titus is to keep emphasizing “these things” in a firm, confident way.
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 3:8, then read the surrounding section so the answer stays connected to the passage's flow.
8This saying is faithful, and concerning these things I desire that you affirm confidently, to the end that they who have believed God may be careful to maintain good works. These things are good and profitable to men;