Many are won by justice and benevolence and courtesy, but all this must be sincere.
95. It is plain, then, that equity strengthens empires, and injustice destroys them. How could wickedness hold fast a kingdom when it cannot even rule over a single family? There is need, therefore, of the greatest kindness, so that we may preserve not only the government of affairs in general, but also the rights of individuals. Benevolence is of the greatest value; for it seeks to embrace all in its favours, to bind them to itself by fulfilling duties, and to pledge them to itself by its charm.
96. We have also said that courtesy of speech has great effect in winning favour. But we want it to be sincere and sensible, without flattery, lest flattery should disgrace the simplicity and purity of our address. We ought to be a pattern to others not only in act but also in word, in purity, and in faith. What we wish to be thought, such let us be; and let us show openly such feelings as we have within us. Let us not say an unjust word in our heart that we think can be hid in silence, for He hears things said in secret Who made things secret, and knows the secrets of the heart, and has implanted feelings within. Therefore as though under the eyes of the Judge let us consider all we do as set forth in the light, that it may be manifest to all.
Source: On the Duties of the Clergy (New Advent)