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.

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

3462. 'So it was on that day' means that state. This is clear from the meaning of 'day 'as state, dealt with in 23, 487, 488, 493, 893, 2788, here the state of the doctrine that is being dealt with.