Their authenticity was quickly questioned by critics, such as John Bruce of the Camden Society, Ralph Waldo Emerson, Thomas Babington Macaulay and Bernard Bolingbroke Woodward.

Carlyle included the letters in the third edition on the advice of John Forster and Edward FitzGerald, despite the addition of a headnote to the appendix stating that they were "semi-romantic or Doubtful Documents of Oliver's History".

In 1885, Samuel Rawson Gardiner discovered evidence which contradicted assertions contained in the Squire papers, though William Aldis Wright still defended them as authentic.

[6] James Anthony Froude called it the nineteenth century's "most important contribution to English history," explaining that "with the clear sight of Oliver himself, we have a new conception of the Civil War and of its consequences.

"[7] George Peabody Gooch wrote that "it was the proudest achievement of [Carlyle's] life to restore to England one of her greatest sons ... the 'Cromwelliad' remains a marvellous production.