Ye are the salt of the erthe. but and if the salt be once unsavery, what can be salted therwith?
1597, William Shakespeare, Romeo and Juliet, V.1:
Noting this penury, to my selfe I said, An if a man did need a poyson now, Whose sale is present death in Mantua, Here liues a Caitiffe wretch would sell it him.