The predictions of the prophets were conditional.
They were made to enforce the appeal for righteousness in the present. They
foretold the consequences of sin on the one hand, and of righteousness on the
other. Judgments might be averted by repentance. Blessings might be forfeited
by disobedience. This principle is clearly laid down in Jer 187-10,
and is of universal application. The ‘if’ is implied even when it is not
expressed. Thus Jonah’s prediction that Nineveh would be destroyed in forty
days was not fulfilled, yet Jonah was not a false prophet, because the threat
was only made on the supposition that Nineveh remained impenitent. Such
predictions, it has been said, were made not that they might, but that they
might not, be fulfilled. (J.R. Dummelow, A
Commentary on the Holy Bible [London: Macmillan and Co., 1909], xliv)