What is the origin of the phrase "History teaches, never trust a Cecil"?

I've come across the phrase "History teaches, never trust a Cecil!" in different places (among others, in the Netflix series "The Crown" with regards to Lord Salisbury).

While the sentiment is easy to appreciate, given the illustrious history of the Cecils as courtiers, I feel that the shared form points to a literary origin of sorts. I just haven't been able to find it.

Could anybody please help?

I have only a rather obvious and unsatisfying bit of research to offer, but as no one has previously cited a contemporaneous news item regarding the expression "Never trust a Cecil" as it was used in 1998 in connection with Lord Cranborne, here is the earliest one that a Google search finds. From "Hague's history lesson," in The Economist (December 3, 1998):

“History teaches, never trust a Cecil,” mused a senior Liberal Democrat earlier this year as the struggle over the future of the House of Lords took shape. William Hague, the Tory leader, may regret not taking that thought on board. For his entire strategy on Lords reform appears to have been ruined by Lord Cranborne, the Tory leader in the House of Lords and a scion of the ancient Cecil family.

Operating largely behind Mr Hague's back, Lord Cranborne cut his own deal on Lords reform with the Blair government. The Tory leader responded by sacking Lord Cranborne as the party's spokesman in the Lords. But if this was intended to reassert his authority in the party, it swiftly went horribly wrong. It soon emerged that most Tory lords were backing Lord Cranborne rather than the putative leader of their party—Mr Hague. Indeed, Mr Hague had been able to persuade Lord Strathclyde to take over as Tory leader in the Lords only by agreeing to let him support parts of the very deal Lord Cranborne had been sacked for negotiating.

A straightforward reading of the opening quip suggests that the speaker wasn't invoking an old aphorism about the untrustworthiness of the Cecils, but drawing a playful (albeit unflattering) extemporaneous connection between the present Lord Cranborne and his slithery ancestors.

Better research than mine may turn up a much older expression along the lines of "A Cecil's word is as good as the Devil's." Until that happens, however, I'm inclined to see the expression as a witty modern-day invention—and very likely the senior Liberal Democrat's one (regrettably anonymous) claim to 15 minutes of fame.

Update (July 5, 2019)

At least one author since 1998 suggests that the truism about the slyness of the Cecils has been around for a while. From Elizabeth Bowen, "Note on Eire": Espionage Reports Winston Churchill, 1040–1942 (1999) [snippet view]:

Ms Bowen worked to Lord Cranbourne who, as a Cecil, was an old family friend. The connection between the Cecils and spying goes right back to Elizabethan times. No doubt, that is the reason for the piece of English wisdom that warns, "Never trust a Cecil".

Still, for a "piece of English wisdom," the warning "Never trust a Cecil" doesn't leave much of a paper trail through the years. Bowen's note would be far more compelling if it had appeared before—rather than a year after—the incident reported in The Economist, althought it's intriguing in any case.

An interesting (and much earlier) remark in Phil Harum, "Pencil Points," in the [Parkes, New South Wales] Western Champion (March 16, 1922) makes the opposite recommendation—"trust a Cecil"—albeit in a rather backhanded way:

In the British House of Commons Lord Robert Cecil secured leave, by a thumping majority, to introduce a bill extending the franchise to women on the same terms as to men. There is no earthly hope of the bill passing into low session, but a beautiful vote catching device it may prove at the coming English elections. Trust a Cecil to assume the virtue of democracy, even though he may have it not.

The notion that "a Cecil" has a certain affinity for the public sphere also appears in "Imperial Parliament: Notes on Careers," in the [Newcastle, New South Wales] Northern Times (November 3, 1916):

For a man born to a private fortune, which rendered any vocation for its emoluments a superfluity, Mr. Balfour proved his natural amiability of disposition by bowing to the will of his uncle and accepting the family tradition that the duty of a Cecil is to concern himself in governing, and, therefore, he entered political life, the turmoil of which was anything but congenial to his nature.

A couple of references to the underhandedness—or political astuteness—of "a Cecil" appear in seventeenth-century sources. From William Lloyd, "A Sermon Preached Before the House of Lords, on November 5, 1680" (1680):

If all this will not affect us with a sense of what we owe to God for his mercy, we are so far from being like Gods ancient People, that we deserve to be given up to strong Delusions, to a belief of Popish Legends, of a Cecil's Plot, and such like sensless Fictions ; which none could give credit to, that had not first subdued his understanding to the belief of any thing, how incredible soever, by the belief of Transubstantiation.

And from An Account of the Late Horrid Conspiracy to Depose Their Present Majesties, K. William and Q. Mary, to Bring in the French and the Late King James, and Ruine the City of London (1691):

What's perhaps more sure, is that as deep as these oles lay, they did not work in the dark, but their Design were known some time since, to some Ministers of State, (at least one) and that few steps they took or Feet they advanced, were Secrets to them against whom they were level'd; which shews us, that the present Government wants not those who can manage things of this nature, perhaps not much less dexterously than a Cecil or a Walsingham, and has further had a large and a happy Influence in opening some persons Eyes, which Prejudice had so long either blinded or changed their colour in relation to the Honour and Integrity of a certain Noble Person, who after all the hot and hasty Censures past upon him, has been the chief or sole manager of this Detection and Discovery.

The expression "a Cecil's plot" may refer to the charge sometimes made that Robert Cecil, the chief discoverer of the 1605 Gunpowder Plot, invented the whole thing. A footnote in John Gerard, What Was the Gunpowder Plot? (1897) gives a snapshot of Robert Cecil's reputation:

Parke and Co. This author says of Cecil and his rival Raleigh, "Both were unprincipled men, but Cecil was probably the worst. He is suspected not only of having contrived the strange plot in which Raleigh was involved, but of being privy to the proceedings of Catesby and his associates, though he suffered them to remain unmolested, in order to secure the forfeiture of their estates" (p. 3338).

In the main text on that same page, Gerard offers the following remarks:

It may be added that amongst modern historians who have given special attention to this period, several, though repudiating the notion that Cecil originated the Plot, are strongly of the opinion that as to the important episode of the "discovery," the traditional story is a fabrication. Thus, Mr. Brewer declares it to be quite certain that Cecil had previous knowledge of the design, and that the "discovery" was a fraud. Lodge is of the same opinion, and so is the author of the Annals of England. Jardine [Criminal Trials (1832)] inclines to the belief that the government contrived the letter to Monteagle in order to conceal the means by which their information had in reality been obtained. Mr. Gardiner [History of England (1883)], though dismissing the idea as "absurd," acknowledges that his contemporaries accused Cecil of inventing the whole Plot.

Conclusions

Prominant Cecils have demonstrated their mastery of political intrigue since the days of Lord Burghley, so it is plausible that there might be (as Elizabeth Bowen, writing in 1999, asserts there is) a "piece of English wisdom that warns, 'Never trust a Cecil'." Nevertheless, I have been unable to find any published instance of that piece of wisdom, expressed in approximately those terms, prior to the 1998 Economist article reporting a senior Liberal Democrat's use of it.

More-general warnings about not trusting proud, ambitious, or selfish men do appear in the last third of the seventeenth century. For example, from Richard Baxter, A Christian Directory, or, A Summ of Practical Theologie and Cases of Conscience Directing Christians How to Use Their Knowledge and Faith, How to Improve All Helps and Means, and to Perform All Duties, How to Overcome Temptations, and to Escape or Mortifie Every Sin (1673):

A proud man makes no bones of any falshood, slander, deceit or cruelty, if it seem but necessary to his greatness, or honour, or preferment, or ambitious ends: he careth not who he wrongeth or betrayeth, that he may rise to his desired height, or keep his greatness: never trust a Proud man further, than his own interest bids you trust him.

And Baxter again, speaking more generally this time, in A Treatise of Self-Denial (1675):

Yea Selfishness makes men false and treacherous, so that they are not to be trusted, and are unmeet materials for any Society. For what ever they promise, pretend, or seem, they are all for themselves, and will be no further true and faithful to the Society, or any Member of it, than suiteth with their own Ends: Never trust a selfish person, if it be your own brother, further than you can accommodate and please him, and so oblige him to you upon his Own account. It is the Complication of Interests, that makes Husband and Wife so much agree and love each other: because that which one hath the other hath: but if their Interests fall out to be any whit divided, it is two to one but selfishness will divide their affections.

So Baxter would undoubtedly endorse the proposition "Never trust a Cecil or any other ambitious or proud or selfish person, or anyone whose interests differ even a whit from your own." That pretty well covers it.

I also think this expression has deep roots. The Cecils' relationship with the English monarchy dates back to a letter written by Princess Elizabeth Tudor to William Cecil in 1548. In the letter she thanks him profusely for some unexplained service. From that, Cecil became Elizabeth's most powerful minister. In the first decades of Elizabeth's reign, Cecil built 3 of the largest estates in England at a cost far greater than his income would permit. This suggests he was either manipulating the Treasury, or extorting large sums from the crown. A libelous (?) pamphlet from that time, titled 'Regnum Cecilianum' describes the situation. Another pamphlet, 'Leicester's Commonwealth', names Robert Dudley as a co-Regent with Cecil during Elizabeth's rule.