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)

10636. 'Because this is a marvel which I am doing with you' means the essential nature of the Word in every single part. This is clear from the representation of 'Moses' as the Word, dealt with in the places referred to in 9372, so that 'a marvel which Jehovah was about to do with him' means that it will be Divine in every single part, as above in 10633.