Theophylact of Ohrid

A disciple of Christ must be "salt," that is, he must not only be good in himself and free from evil, but must also communicate goodness to others. For such is salt. It, itself remaining unharmed and free from corruption, preserves from corruption other things to which it imparts this property. …

Commentary on Luke ↗

Gregory the Dialogist

For we must continually weigh what is said to the holy apostles, and through the apostles to us: You are the salt of the earth. If therefore we are salt, we ought to season the minds of the faithful. You then, who are shepherds, consider that you are feeding God's animals. Concerning these animals indeed it is said to God through the Psalmist: Your animals shall dwell in it. And we often see that a block of salt is set before brute animals, so that they may lick that same block of salt and be improved. …

Forty Gospel Homilies, Homily 17 ↗

Bede

He had said above that the tower of virtue was not only to be begun, but also to be completed, and to this belongs the following, Salt is good. It is a good thing to season the secrets of the heart with the salt of spiritual wisdom, nay with the Apostles to become the salt of the earth. (Matt. 5:14.) For salt in substance consists of water and air, having a slight mixture of earth, but it dries up the fluent nature of corrupt bodies so as to preserve them from decay. …

Catena Aurea by Aquinas ↗

Bonaventure

Second, as regards caution against a harmful thing, he adds: Salt is good, but if the salt loses its savor. By salt is understood every perfect disciple, who ought to season the hearts of others by example and word; whence Matthew 5: "You are the salt of the earth"; which is said because he ought to have the highest discretion in conduct, according to that passage of Colossians 4: "Let your speech always be seasoned with salt in grace"; because, just as food cannot be eaten unless it is seasoned with salt, so neither does the stomach …

Commentary on Luke, Chapter 14 ↗