The Conferences Of John Cassian by John Cassian
Chapter XXIX. How we ought to do those things which are to be kept secret.
You must also provide with the same care that if by chance some word has slipped out of your mouth which you want to be a secret, no injunction to secrecy may trouble the hearer. For it will be more likely to be unheeded if it is let pass carelessly and simply, because the brother, whoever he is, will not be tormented with such a temptation to divulge it, as he will take it as something trivial dropped in casual conversation, and as what is for this very reason of less account, because it was not committed to the hearer's mind with a strict injunction to silence. For even if you bind his faith by exacting an oath from him, you need not doubt that it will very soon be divulged; for a fiercer assault of the devil's power will be made upon him, both to annoy and betray you, and to make him break his oath as quickly as possible.