Of course, the field of music theory is quite broad ranging from scales to rhythms, intervals, chords, modulations, formal procedures, ornamentation etc. To learn everything takes quite a bit of time, usually at least up to 36 weeks of intense studying.
But are there any minimum skills in music theory which would be absolutely necessary to learn but would not take as much time? In other words, what would happen if we learned only the most practically needed concepts in a few weeks first and leave the more advanced stuff for later study as we move forward?
I think this approach is quite valid and practical for many of today's aspiring organists. Here are the few key things they would need to learn first in music theory:
1) Half-step and whole-step
3) Major and minor scale
4) Stable and unstable steps of the scale
5) Diatonic intervals of the natural major and minor scale (up to an octave)
6) Circle of fifths
8) Tonic, Dominant, and Subdominant chords and their inversions
I'm including all these elements of music theory in Weeks 1 and 2 of my Harmony for Organists Level 1 course. They will serve as an introduction and preparation for training in four-part harmony and voice-leading.
By the way, do you want to learn my special powerful techniques which help me to master any piece of organ music up to 10 times faster? If so, download my video Organ Practice Guide.