If you haven't done so yet, read my Introduction to the Allied Masonic Degrees, to which this post is a follow-up.
The structure of the Temple is completed, and only the sacred utensils remain to be completed. (Utensils here includes furniture, including the table of shewbread, lampstand, incense altar, etc.)
Having proven himself capable, by way of the plans presented in the preceding degree, the candidate is now recognized for his achievement by being made Chief Architect, thus becoming the successor to Hiram Abiff.
Thematically, this degree feels similar to the Capitular degree of Most Excellent Master, in that it also acknowledges the completion of the Temple.
The Cryptic Mason will find particular interest in comparing this degree to the Royal Master Degree, in which is given a different (and presumably conflicting) story of the appointment of Hiram Abiff's successor.
Masons are, in some measure, familiar with the concept of self-identifying as "Hiram," but this degree is very humbling in that it appoints the candidate in his stead, by merit of his work and achievements. I don't think it is suggesting that the candidate has surpassed Hiram, particularly since he is not in possession of the Lost Word, but rather, that for want of a Chief Architect the Candidate is found to be the best suited for the position.
Steps to the Endowment.
5 months ago