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)

9469. 'And fine linen' means the truth from this, that is to say, from the good which has a celestial origin. 'Flax' or 'linen' means truth, see 7601, and 'fine linen' means truth from the Divine, 5319; both kinds of truth exist in the natural man. 'Fine linen' means truth from a celestial origin because of its whiteness and softness.