112 Masters, command your servants not as being below you in rank, but as remembering that they are sharers of the same nature as yourselves. Servants, serve your masters with good will, for each ought patiently to support that to which he is born, and be obedient not only to good but also to froward masters. For what thanks has your service if you zealously serve good masters? But if you thus serve the froward also you gain merit; for the free also have no reward, if when they transgress they are punished by the judges, but this is their merit to suffer without transgressing. And so you, if contemplating the Lord Jesus you serve even difficult masters with patience, will have your reward. Since the Lord Himself suffered, the just at the hand of the unjust, and by His wonderful patience nailed our sins to His Cross, that he who shall imitate Him may wash away his sins in His Blood.
Source: Letters (New Advent)