It is commonly believed that variation 25 is the emotional and spiritual center of Bach's Goldberg variations. In the course of learning to play the Goldberg Variations in it's entirety, I believe that a case can be made that variation 27 is in fact the spiritual culmination of the Goldbergs.
Sequentially, it is the final canon of the Goldbergs which form the consistent thread of the progression of the Goldbergs in groups of 3. It is the final canon of the series.
More importantly, the 25th is a message of the perception or experience of suffering in it's unmitigated clarity. The essence of life is suffering and this experienced knowledge is expressed in the 25th. But that is not the end of the matter because life is not purely object. It is not merely an "is", however described. Therefore the 25th cannot be the culmination of existence which is not purely objective. It still leads up to something else.
Agency is ineffable. It is not a thing. There is no object to which it can be absolutely bound or explained, neither meaning or ethics despite being the subject of those. The 27th, which in my view is about agency in it's entirety (encompassing the object) and therefore the unique nature of human experience, is not about a thing, even though it integrates all that comes before it. It could only arrive in authenticity after the 25th, which like all the variations and canons prior, leads up to it.
The absence of objecthood is signified by the absence of ground in the third voice in the bass, unique among the canons. The structural removal of the ground signifies that agency is without object or reference point, such as an externalized purpose or even an identifiable self (in the Humean/ Buddhist sense). Without object, one cannot posit a soul or a God. Guidance cannot be external, or oriented towards an object - the reference point or pole star is absent. The haunting interval of the 9th is the breadth of the existential gap. This gives it a specific kind of absence.
Peter Williams points out the seemingly redundant and isolated nature of the 27th, in that it is a second canon with the second interval and there is no further second canon in the third. In this regard, the numerical significance of the canon should be noted. There is a sense in which Bach had extended the canons to include one in the cube of 3, a reference to ninth hour of Christ, the moment of a perceived abandonment by God. This is the moment of existential crisis (as a kind of self-consciousness) itself percolating through the cosmos. But that is precisely the significance of Jesus as the creation principle, which must face the absence of that out of which it creates, as the ground of agency.
The 27th is fairly earnest, particularly the parallel ascending/descending patterns in the left and right hand. This is not innocence. There is a knowledge of tragedy expressed but a drive to move forward which reflects not hope but the curative nature of agency which is essentially pure and childlike, and is this untouched (if not unaffected) by the immense suffering before it. This makes agency possible, and is the gift of God which Bach expresses. The spiritual nature of the message is underlaid by the following left or right hand after the note on the first beat, a common figure in sacred music.
The second section proceeds to an exploration of ineffable and unknowable nature of agency, which brings uncertainty and the unexpected. This is received in an ambiguous manner, corresponding to the descending guiding bass notes. As it were, one has to be on one's guard. This allows for a degree of open-endedness to the interpretation not found in the first section.
Commentators have pointed out the grounded and human nature of the Quodlibet which is again a departure from the canons. Following from the 27th, the work is no longer at the level of structure but of agency, and is this understandably expressed in humanly grounded rather than abstract works.
The 28th to the 30th is much lighter than the rest of the Goldbergs. The spiritual work is done. The need for teleological resolution has been encountered in it's entirety, even if not resolved or removed. Levity after consciousness is permitted and the weird and wonderful is indicated, before the final return home.