Here is a Russian icon of Ioann Zlatoust — John the “Golden-mouthed,” better known as John Chrysostom:
We are today primarily interested in the text on the book he holds:
Here it is, with the portion seen on the book in bold type:
Вы есте соль земли: аще же соль обуяет, чим осолится? Ни во чтоже будет ктому, точию да и-[зсыпана будет вон и попираема человеки.]
It is the text of Matthew 5:13:
“You are the salt of the earth. If however the salt loses its strength, how shall it be salted? It will then be good for nothing but to be t[hrown out and trampled by men.]”
Perhaps you noticed some slight spelling variations in the text. These are common in old inscriptions.
Now why should this text be included on an icon of John Chrysostom? Well, most likely because he included a discussion of it in one of his homilies:
“What then? did they restore the decayed? By no means; for neither is it possible to do any good to that which is already spoilt, by sprinkling it with salt. This therefore they did not. But rather, what things had been before restored, and committed to their charge, and freed from that ill savor, these they then salted, maintaining and preserving them in that freshness,  which they had received of the Lord. For that men should be set free from the rottenness of their sins was the good work of Christ; but their not returning to it again any more was the object of these men’s diligence and travail.”
John seems not to have understood the original meaning of the saying, and in fact there is much controversy even today about what was meant by salt losing its strength or savor, because salt remains — well, salt, no matter how old it is. It does not lose its strength or savor. Some think that the “salt” mentioned was not at all what we know as salt, but rather a kind of substance used to fertilize the fields — a fertilizer that could lose its strength. But no firm and definitive solution to the puzzle of this text seems yet to have been found — so it remains obscure.