The "midst" of something in the Bible represents the thing that is most central and most important to the spiritual state being described, the motivation that drives everything else. In general this will be something we love or feel, because at the core of things we are what we love; our loves define us.

(Reference: Apocalypse Revealed 44, 90, 268; Arcana Coelestia 2252, 2940, 10153, 10365, 10557, 10635, 10641)

Plants in general stand for facts and ideas, so it makes sense that a forest in the Bible represents a collection or system of external, factual, ideas. This can be a collection on its own, or it can be the collected external knowledge of a church. It can also represent the level of a person's mind where he or she deals with such systems of ideas.

(Reference: Arcana Coelestia 9011 [1,3-7], 9642 [3,23]; The Apocalypse Explained 650 [26,49])