"I am the Knowledge of my Name."
— Thunder, Perfect Mind
In the Golden Dawn the magical motto is not just a fancy name or title, but a representation of the spiritual and magical goals of the magician. It is, in many ways, the first magical act performed by the Candidate, consecrating themselves towards a given purpose, redirecting their life as they see fit.
The motto therefore carries a lot of power, particularly when employed regularly in ritual work, where it becomes the very embodiment of the magical persona of the initiate. On entering the temple, Bob is no longer Bob, but Frater Whatever, and if he really chose that as his motto we would expect to see disinterest and apathy become a central focus and defining factor in his life.
So, with so much wrapped up in our magical name, should we keep it secret or let the whole world know? There are varying schools of thought on this, none of which are right or wrong, and ultimately the choice to share your motto is a personal decision to make, just as the choice of the motto itself should be personal (and not assigned to you, like the policy of certain groups).
Some view the motto as one of those things that should be kept secret, but the reasons for this secrecy are often vastly different.
On one hand, secrecy is a well-known aspect of the occult, both for mundane and magical reasons. It has been written about in many tomes how secrecy can aid our magical work. It is often associated with the idea of containing power, just as a magician might not tell anyone about a magical working until it has been completed and achieved, lest the generated energy and intent towards that goal is dissipated. Some of this line of reasoning has even become part of pop culture in the form of birthday wishes, which must not be shared or they won't come true.
On the other hand, some believe that sharing their motto, which they think of as their true name, will give people power over them. There is a long-standing tradition that supports this idea, with occultists regularly gaining power over spirits through the employment of the spirit's name, or a certain divine name. This approach even made it into the well-known Rumpelstiltskin fairytale, where the woman was able to escape her bargain with the imp by telling him his name. The importance of names and their power is also explored frequently in the Qabalah.
In the Golden Dawn we see this coming into play when the Candidate is challenged by, for example, the Hiereus in the Neophyte ceremony: "Thou canst not pass by me, saith the Guardian of the West, unless thou canst tell me my name."
Thus, some people claim that sharing their motto with someone else gives the other person, or people, power over them. Of course, a counter to this argument is that usually a motto is shared within an Order, which would, by this logic, give fellow initiates power over them, and if a magician really lost his or her power by sharing their motto I would personally question how much of a magician they were in the first place.
On the third hand (which every occultist should really have), there are the various oaths of secrecy to take into account. Some magicians hide their motto because they have been or believe they have been sworn to keep it secret, with little consideration for any possible magical reasons whatsoever. In most Golden Dawn groups, however, the Neophyte Oath does not list the personal motto of the initiate as something that must be kept secret, and even the requirement not to share the names of fellow members does not specify their mottoes (though some would argue that these are their true names and thus fall under this line of the oath).
The opposing viewpoint to the employment of secrecy is the idea of revelation, of sharing one's magical motto far and wide. This might be a wilful act, where the magician deliberately employs their motto (such as in the signing of a charter or initiation certificate, the inscribing of the motto on a talisman that will be seen by others, or as I have done in the image above), or it could be a simple refusal to accept that there is anything wrong in sharing such a name.
If you haven't guessed by now I find value in both approaches, as there is certainly power in secrecy and power in names, but my experience has shown me that in revealing one of my mottoes I have not been weakened or subjected to the rulership of another. In fact, in many ways I have been strengthened and empowered, because every time someone speaks to or of me when employing my motto, they identify me with my magical goal, constantly affirming the name and empowering the act of magic that went behind it.
"I am the Utterance of my Name."
— Thunder, Perfect Mind