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.

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