We have all heard the famous quote attributed to St. Francis:
Preach the Gospel at all times. Use words when necessary.
St. Jerome makes mincemeat of that nonsense in Lettter 69 to Oceanus.
As regards the obligation of priests to be teachers we bare have the precepts of the old Law Deuteronomy 17:9-11 and the fuller instructions given on the subject to Titus. Titus 1:9-14 For an innocent and unobtrusive conversation does as much harm by its silence as it does good by its example. If the ravening wolves are to be frightened away it must be by the barking of dogs and by the staff of the shepherd.
https://www.newadvent.org/fathers/3001069.htm
An innocent conversation does no good if it is not accompanied by the preached doctrine of the Church. That is what drives away the "ravening wolves" such as heretics and apostates who would seek to destroy the Church. After all, how does an innocent conversation show forth the correct doctrine of the Trinity or the correct doctrine of the Person and work of Jesus Christ? It does not. Therefore preaching is very much a necessity.
