in the middle of her; she will not be moved ... help her

The words "her" and "she" refer to "the city of God."

she will not be moved

Here, the word "moved" is the same word translated as "shaken" in Psalms v. 2. The writer speaks of the destruction of Jerusalem by armies as if an earthquake were to destroy it. This can be stated in active form. Alternate translation: "nothing will be able destroy her" (See: rc://en/ta/man/translate/figs-metaphor and rc://en/ta/man/translate/figs-activepassive)