When a prophet speaks in the name of the LORD, if the thing follows not, nor comes to pass, that is, the thing which the LORD has not spoken, but the prophet has spoken it presumptuously: you shall not be afraid of him.
Read Chapter 18
George Leo Haydock
AD 1849
Fear him. Septuagint, "you shall not spare him "but cut him off by death. (Haydock)
Though the completion of what has been foretold be not a sure sign of a true prophecy, (chap. xiii. 2,) yet when the thing does not come to pass which the prophet had spoken unconditionally, he must undoubtedly be rejected; as also when he speaks in the name of false gods. No miracles can then establish his credit. The prophecy of Jonas, and many of the other prophecies, were conditional. (Calmet)