The teaching is actually in more than one letter:
Titus 1:5 ¶ For this cause left I thee in Crete, that thou shouldest set in order the things that are wanting, and
ordain elders in every city, as I had appointed thee:
Titus 1:6
If any be blameless, the husband of one wife, having faithful children not accused of riot or unruly.
Titus 1:7 For a bishop must be blameless, as the steward of God; not selfwilled, not soon angry, not given to wine, no striker, not given to filthy lucre;
Titus 1:8 But a lover of hospitality, a lover of good men, sober, just, holy, temperate;
Titus 1:9 Holding fast the faithful word as he hath been taught, that he may be able by sound doctrine both to exhort and to convince the gainsayers.
It is more serious than if Paul is just teaching his own opinion because it appears in more than once place so you would have to disregard Paul.
2 Thessalonians 3:14 Take special note of anyone who does not obey our instruction in this letter. Do not associate with them, in order that they may feel ashamed.
Paul also calls it sound doctrine.