in Dryden's satire of Absalom and Achitophel, is meant for Oliver Cromwell. As Saul persecuted David and drove him from Jerusalem, so Cromwell persecuted Charles II. and drove him from England.
“They who, when Saul was dead, without a blow Made foolish Ishbosheth [Richard Cromwell] the crown forego.” Part i. lines 57, 58.
Saul among the prophets?
The Jews said of our Lord, “How knoweth this man letters, having never learned?” (John vii. 15.) Similarly at the conversion of Saul, afterwards called Paul, the Jews said in substance, “Is it possible that Saul can be a convert?” (Acts ix. 21.) The proverb applies to a person who unexpectedly bears tribute to a party or doctrine that he has hitherto vigorously assailed. (1 Sam. x. 12.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894