From the great John Chrysostom, via Sunday’s Office of Readings:
Do not think, he says, that you are destined for easy struggles or unimportant tasks. You are the salt of the earth. What do these words imply? Did the disciples restore what had already turned rotten? Not at all. Salt cannot help what is already corrupted. That is not what they did. But what had first been renewed and freed from corruption and then turned over to them, they salted and preserved in the newness the Lord had bestowed. It took the power of Christ to free men from the corruption caused by sin; it was the task of the apostles through strenuous labor to keep that corruption from returning.
If others lose their savor, then your ministry will help them regain it. But if you yourselves suffer that loss, you will drag others down with you. Therefore, the greater the undertakings put into your hands, the more zealous you must be. For this reason he says: But if the salt becomes tasteless, how can its flavor be restored? It is good for nothing now, but to be thrown out and trampled by men’s feet.
When they hear the words: When they curse you and persecute you and accuse you of every evil, they may be afraid to come forward. Therefore he says; “Unless you are prepared for that sort of thing, it is in vain that I have chosen you. Curses shall necessarily be your lot but they shall not harm you and will simply be a testimony to your constancy. If through fear, however, you fail to show the forcefulness your mission demands, your lot will be much worse, for all will speak evil of you and despise you. That is what being trampled by men’s feet means.”