Now, this letter is not obscure, hidden, or unknown. It has been in print for two hundred years, and the full text is below with the relevant passage in bold:
Be it kend till all men be thir present letres, ws Marie Queen Dowarere of Scotland, to be bundin and oblist, and be thir present letres bindiss and oblisses us, to ane honourable man and our well belovit Sir William Sinclar of Roflin, knyt: Forasmeikle as the said Sir William is bundin and oblist to us, in specials service and manrent, for all the days of his life, to gang and ryde with us, and to tak our sauld part with his kyn, servandis, and freyndis, that will do for him contrare and against all that leiff or denay his allegiance to the crowne of Scotland, and authority thereof allenarly exceptit, as at mare'length is containit in the said Sir William's band made to us thereupon; herfor we bind and obliss us to the said Sir William, in likwis that we sall be leill and true maistres to him, his counsell and secret shewen to us we sall keip secret, and in all mattres gif to him the best and trewest counsell we can, as we sall be requirt therto, and fall not witt his stealth nor damnage, but we sall stop it att our power, and sall tak his aiked and plain part, maintain and defend him be ourself, our penssionaris, servandis, partaikers and assistants, that will do for us, in all his actions, cauiles and querrils, contraire and against all men that leive, or denay the crowne of Scotland and authority thereof, being allenarly except, and we sall be readie att all time to maintain and defend him, as said is, als aft as we sall be requirt thereto, be ourself, our men, freyndis, assistants, and partakaris with us, and all that we may parches, wytbout dissimulation, fraude or gile, and generallie we sall do all that pertens, or is knawne to pertane to ane Maistres in the manteyning and defending of hir men and servandys; and attour, for the gud, faithfull, trew and thankfull service done and to be done to us be the said Sir William, we have given and grantit, and be thir our present letres gewis and grantis to the said Sir William, ane yeirlie pensioun of the soume of three hundreth mark is, usual money of Scotland, to be payit to him yeirlie, dureing his and our lifetyme, att twa termis in the year, that is to say, Whitsunday and Mertimes in winter, be equale portions, begynand the sirst payment att the fest of Whitsunday, in the year of God 1546 yeire, and binds and obliss us, that within the space of ane yeir next to cum we sall gif the said Sir William, assignation of the males or ferms of our landis in competent place, whereof he may get yerely thankfull payment of his said pension of three hundredth markis att the termis above written. In wittness of the quhilk thing, to thir present letres subscrivit with our hand, our signet is affix it, att Striveling the third day of June, the yeir of God 1546 yeirs.
From this they spin a tale of esoteric secrets, ignoring the clear context of the letter, which is a standard feudal obligation and grant of pension.
This is repeated in in Carol Schaefer’s 2002 biography Mary, Queen of Scots: A Spiritual Biography, where the author imagines that the Scottish royals were intimately involved in anti-Catholic esoterica.
Philip Coppens, in The Stone Puzzle of Rosslyn Chapel (2004, p. 23) wrote that Mary of Guise, promised to be loyal to William Sinclair after seeing “a great secret within Rosslyn,” which he placed in quotation marks and attributed to a “1545” letter from Mary to William. It was obvious that Coppens never read the letter; nevertheless, his words were repeated as fact in the 2004 book Guardians of the Holy Grail by Mark Amaru Pinkham and the 2006 novel Edinburgh Knights by Elaine Pomm, as well as the 2012 nonfiction book Da Vinci’s Last Commission by Fiona McLaren.
When Alan Butler and John Ritchie quoted it in this year Rosslyn Chapel Decoded, it had moved back to 1546 and read quite differently:
We bind and oblige ourselves to the said Sir William, and shall be a loyal and true mistress to him. His counsel and secret shown to us we shall keep secret, and in all manners give to him the best and truest counsel we can, as we shall be required thereto.
Let’s take the quotation part by part.
First, “bind and oblige” is a legal term in use down to the modern era in the British Isles, and is also found in early American documents. It was the common form of writing a contract. It is neither special nor bizarre. In this case, it is used as part of a fealty oath. Consider, for example, the letter of fealty provided in the opposite direction from James Earle Douglas to James II: “I bind and oblige me till our said soverayne lord…” Feudal bonds worked in two directions, and the king provided a similar pledge to oblige himself as master and protector of the vassal. The Holy Roman Emperor Otto I, for example, told his vassals “now you shall be mine.” Such oaths were nearly a thousand years old.
Do I even have to explain that “mistress” is not a term of “submission” but is rather the feminine form of master? Would anyone accuse a man who said he was master of another of being subservient? Mary pledged to be a loyal and true mistress (i.e. female master) to William. This is an assertion of supremacy entirely in keeping with feudal oaths of obligation and fealty.
In the next line the word “secret” is the key element causing trouble for Sinclair speculators. Here Mary is using “secret” in the older sense, derived from Middle English usage, whereby it means “a confidence.” In other words, the “secret” is part of the “counsel” and refers to confidential advice that Mary promises not to make public, and also promises to repay his counsel with that of her own. It cannot, given the context, refer to the Holy Bloodline of Jesus or the Holy Grail, in which case it could not logically fit in a sentence devoted to legal obligations and pension payments—you know, the boring stuff of government. After all, if it was an esoteric secret, why write it in a formal letter of state entered into the Scottish royal archives?