Proverbs 18:19

A brother offended [is harder to be won] than a strong city,
&c.] A fortified city may sooner be taken by an enemy, than one brother offended can be reconciled to another; their resentments against each other are keener than against another person that has offended them; and their love being turned into hatred, it is more bitter; and it is more difficult to compose differences between brethren than between enemies; wherefore such should take care that they fall not out by the way: this is true of brethren in a natural sense; as the cases of Abel and Cain, Jacob and Esau, Joseph and his brethren, Amnon and Absalom, and others, show; and of brethren in a spiritual sense, as Paul and Barnabas, Luther and Calvin, and others; and [their] contentions [are] like the bars of a castle:
which cannot be easily broken or cut asunder: so contentions, especially those among brethren, are with great difficulty made to cease, and their differences composed; they will stand it out against one another as long as a strong city, or a barred castle, against an enemy.