Ye are the salt of the earth; but if the salt have lost his savour, wherewith shall it be salted: it is thenceforth good for nothing, but to be cast out, and to be trodden under foot of men.
I was reminded of a quote I read in my Book of Mormon class.
“Salt will not lose its savor with age. Savor is lost through mixture and contamination. Similarly priesthood power does not dissipate with age; it, too, is lost through mixture and contamination.” 
Another commentary on this is:
“In an age before refrigerators, salt was the great preservative. In this memorable metaphor, Jesus calls his disciples salt; They would preserve his teachings and lifestyle among the peoples of the earth, but if they failed in their conscientious following of his example, they would be worthless in his kingdom and would be cast out, as useless salt is cast out.”
This makes it more understandable why it says “if the salt have lost HIS savour.” He is, in parable form, speaking about the apostles.
While this is a great metaphor in speaking to them, it is also a cautionary tale for us as members of the church and as Christians. It is important that we not allow “mixture and contamination” by the philosophies of men to become incorporated into the doctrine or our beliefs.
What a blessing it is to have the words of our Savior preserved for us by the hand of his disciples.