“You are the salt of the earth. But if the salt loses its saltiness, how can it be made salty again? It is no longer good for anything, except to be thrown out and trampled underfoot." (Mathew 5:13)
Here is Blessed Anne Catherine Emmerich's interpretation of this passage:
I saw also that the Apostles were sent over the greater part of the Earth to crush the power of Satan and to scatter benedictions. It was just those regions into which they went that had been most thoroughly infected by the evil one. Jesus, by His perfect atonement, acquired that power against Satan for such as had received or such as would receive His Holy Spirit, and He secured it to them forever. I was given to understand that the power to withdraw various regions of the earth from Satan's dominion by means of a blessing, is signified by the words, "You are the salt of the Earth." For the same reason is salt one of the ingredients of holy water. (from the Complete Visions of Anne Catherine Emmerich, p. 12)