There's a longstanding tradition in religions of having an "outer mystery" and an "inner mystery." The gnostics were famous for this. Anyone is invited to learn about the outer mystery, and enjoy those rites, but only the privileged are inducted to learn the inner mysteries. These inner mysteries are subtle enough that even if you were told them, you wouldn't realize they were important until you were ready.
As such, the seven gods may be seen as simply caricatures of the many faces of the one true god. To teach the citizens of the nations, it is effective to provide them with all seven faces, but once you enter the priesthood, you realize you only need one.
As an example, a Chinese martial arts teacher may teach based on the 5 elements: water, wood, fire, earth, and metal. Much of natural phenomena can be described like this, and it is quite easy to understand. I can teach someone the rudiments of it in about 30 minutes.
The Chinese also have a concept of yin and yang. These two polar opposites are constantly interplaying. Despite there being only two concepts, yin and yang, they are much harder to truly capture. I might give a good hearty try at trying to teach yin and yang. After a few weeks I might feel like I got some where.
Deeper still is the concept of wuji - without a ridgepole or without bounds. Wuji is a concept of emptyness and nothingness. The concept of wuji is fundamental to much of the Daoist ideals which lie at the root of their philosophy and martial arts. I would not try to teach wuji. An understanding of it comes with time. If one understands wuji and several of its related concepts like wu wei, in 10 years time, it is a stellar accomplishment.
As such, if the priesthood taught lessons along these lines, there is no point in teaching wuji to the citizens. They simply will not understand it, and it really would not help them in their daily lives. Teaching the 5 elements is far more valuable. Yin and yang may be useful. Let them merely play lip service to wuji. The priesthood, on the other hand, may live and breathe yin and yang, and be on a path to wuji.
We see this in many walks of life where mastery is called for. For example, a jazz musician might teach chords, talking about V7 chords and diminished chords, and all that jazz. But when you look at what they really play, what they practice, the chords fall away. They just play music. It's one thing, and it's all they do. If you ask them what chords they played, they may be able to put together some chord progression for you. If you ask them what chords they will play, they'll just shrug and say "I'll play the music."