8 “God is in the midst of her: she shall not be moved”. Let the sea rage, the mountains shake; “God is in the midst of her: she shall not be moved.” What is, “in the midst of her”? That God stands in any one place, and they surround Him who believe in Him? Then is God circumscribed by place; and broad that which surrounds, narrow that which is surrounded? God forbid. No such thing imagine of God, who is contained in no place, whose seat is the conscience of the godly: and so is God's seat in the hearts of men, that if man fall from God, God in Himself abides, not falls like one not finding where to be.
For rather does He lift up you, that you may be in Him, than so lean upon you, as if you withdraw yourself, to fall. Himself if He withdraw, fall will you: yourself if you withdraw, fall will not He. What then is, “God is in the midst of her”? It signifies that God is equal to all, and accepts not persons. For as that which is in the middle has equal distances to all the boundaries, so God is said to be in the middle, because He consults equally for all. “God is in the midst of her: she shall not be moved.”
Wherefore shall she not be moved? Because God is in the midst of her. He is “the Helper in tribulations that have found us out too much. God shall help her with His Countenance.” What is, “with His Countenance”? With manifestation of Himself. How manifests God Himself, so as that we see His Countenance? I have already told you; you have learned God's Presence; we have learned it through His works. When from Him we receive any help so that we cannot at all doubt that it was granted to us by the Lord, then God's Countenance is with us.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)