John Lorne Campbell has described Forbes as, "an Episcopalian clergyman and ardent Jacobite who later became bishop of Ross and Caithness, and who made it his life's work to collect all the reminiscences of participants in the 1745-6 rising as he could.
"[1] Historian John S. Gibson wrote, that the discovery of Bishop Forbes' research bound together into ten volumes in the library of a Scottish country house during the 1830s was, "alas, just too late for Sir Walter Scott.
In 1735 he went to Edinburgh, was ordained priest by Bishop David Freebairn, and was shortly appointed minister of the non-jurant episcopal congregation at Leith, a town which was to remain his home for the rest of his life.
On 7 September 1745, when Charles Edward Stuart was on his descent from the Highlands, Forbes was one of three episcopal clergymen who were arrested at St. Ninians, near Stirling, suspected of intending to join the Jacobite Army.
An account of the interview that ensued is preserved in his third ‘Journal.’ He made no submission, but thought it better to have his services without singing; and, receiving advice from a friend, he went for some weeks to London.
[6] The bishop permitted favoured guests to drink out of Prince Charlie's brogues; Rachel sent to the ‘royal exile’ the seed-cake which Oliphant of Gask presented to him.
In the bishop's own lifetime appeared An Essay on Christian Burial, and the Respect due to Burying-Grounds, by a ‘Ruling Elder of the Church of Scotland’ (1765), and an Account of the Chapel of Roslin (1774).
His major contribution to history is the ‘Lyon in Mourning,’ ten octavo volumes in manuscript, bound in black, and consisting of highly important primary sources related to the Jacobite Rising of 1745 and Hanoverian atrocities during its aftermath.
The volumes date from 1747 to 1775; the first extracts were published (1834) under the title of Jacobite Memoirs, by Robert Chambers, from the originals in the Advocates' Library, Edinburgh.
It still remains a very painful and even "unpalatable" subject among many Scottish nationalists, however, that the three Hanoverian officers whose names appear most often in accounts by Forbes' informants of the most horrifying alleged violations of the laws and customs of war against Jacobite POWs and the civilians of the Highlands and Islands and, "who are the most bitterly remembered" - Captain John Fergussone, Major James Lockhart, and Captain Caroline Frederick Scott - were all fellow Scotsmen.