I'm looking for a noun that can fit well in a 3-level scale: apprentice, ____, master.
The scale describes the progress of a person from a layman/uninitiated (apprentice), through being accomplished and versed in the secret knowledge he acquired, and finally arriving at the ability to teach and mentor apprentices into the same knowledge (master).
The noun initiate has connotations of being a beginner and thus isn't good for my need.
Journeyman would feel like the most natural choice to me.
(Addendum: Freemasonry goes with fellow craft between entered apprentice and master mason, but that's a bit wordy for my tastes.)
It sounds like adept may meet your needs. The Hermetic Order of the Golden Dawn initiatory structure has 10 grades divided into three tiers, and the middle tier is the adept grades (5=6 adeptus minor, 6=5 adeptus major, and 7=4 adeptus exemptus).
Honestly, I would also be perfectly comfortable using initiate for your situation, not really feeling it very strongly has the beginner implications you mention. Adept clearly has a much stronger connotation of competence, though.
If you were to go with this option for your middle grade, I would also suggest thinking about using initiate or neophyte rather than apprentice for your initial grade, to distance a bit further from the traditional guild apprentice-journeyman-master structure. In fact, drawing a bit further from the Golden Dawn, in which the next grade up from the adept tier is 8=3 magister templi ("Master of the Temple"), we could get a super-sexy progression of neophyte-adept-magister.
Whenever I hear secret knowledge, naturally I immediately think of Freemasonry.
So, I understand there are three degrees of Craft or Blue Lodge Freemasonry (info via Wikipedia):
Therefore, fellow seems appropriate here. You may simply go with member.
Another word that may fit is journeyman. Might have problems with gender neutrality on that one, though.
Just for fun, I'll throw in another word, based on the question's tag line more than on the content: Acolyte
I would not go with Journeyman in spite of its timeless use in guilds as an expert who is not a master. In my opinion its application is more akin to Artisan or Craftsman or someone else who, while still considered an expert, can have varying skill levels from person to person bearing this title. Even a world class glass-blower can be considered a journeyman in my opinion.
Adept, I think, is a generally good middle-grade word since the subject is secret knowledge. I think there are other options available to you, provided you consider bumping apprentice down to something more unskilled (where fun words come in, such as Probationer or Novitiate). However, the context is very limited; secret knowledge can be a little ambiguous.
If the secret knowledge pertains to a skill or technique, I'd go with Practitioner for your middle-grade because of the phrase licensed practitioner which is reminiscent of black-belt level students in a martial arts studio: They are experts that carry some influence over the new Initiates, Novices, Acolytes, or Neophytes while still learning and answering to a Master or Doyen (I really like doyen: a combination of technical expert and leading authority, the perfect hybrid of technical prowess and extensive understanding while still maintaining the Educator status).
For secret knowledge that is intended more for understanding than application, you might [carefully] consider Graduate since this intermediate level of knowledge is mimicked in modern education. Somebody who is not a novice anymore (out of college) may continue their learning and get a Master's degree! (coincidence?)
However, use caution in immediate context because since many people choose not to continue their education, Graduate has acquired a connotation of culmination or conclusion. So when introducing Graduate as a tier, be sure there is clear reference to a master figure. Scholar also accomplishes this, while a little weaker in my opinion.
How about practitioner? It is “a person actively engaged in an art, discipline, or profession” (New Oxford American Dictionary).
I talked with my Boilermaker friend yesterday to prepare for this search. He says the Boilermakers Union (NOT their full name) currently recognizes (at least informally) 5 levels: Helper, apprentice, journeyman, foreman and instructor. They've moved away from the title master.
A helper provides the non-technical assistance needed to create a "boiler." A helper lifts, carries, finds tools or holds parts during construction.
An apprentice is learning to perform the technical aspects of construction. An apprentice is closely supervised, and participates in formal instruction provided by the instructors.
A journeyman has completed the apprenticeship program and is trusted to complete all aspects of construction under the considerably less focused supervision of the foreman.
For my secret knowledge/religious purpose, I'm using: Initiate, devotee, adept, practitioner and master.
Expert is commonly used as a skill level between apprentice and master.
Also, disciple or mentor.
