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.

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

352. 'The firstborn of the flock' means the holy which is the Lord's alone. This becomes clear from the firstborn in the representative Church, where everything was holy because it had regard to the Lord, who alone is 'the Firstborn'. Love and faith deriving from it are the firstborn. All love is the Lord's and not a trace of it man's, and this is why the Lord alone is 'the Firstborn'. This was represented in the Churches of the ancient era by the requirement to consecrate the firstborn of man and of beast to Jehovah, Exodus 13:2, 12, 15. It was also represented by the tribe of Levi, which in the internal sense means love - though Levi was born later than Reuben and Simeon, who in the internal sense mean faith - being taken instead of all the firstborn and becoming the priesthood, Numbers 3:40-46; 8:14-20. That the Lord as regards His Human Essence is the Firstborn of all is clear from the following statement in David,

He will cry to Me, You are My Father, My God, and the Rock of My Salvation; I will also make Him as the Firstborn, supreme over the kings of the earth. Psalms 89:26-27.

And in John,

Jesus Christ, the Firstborn from the dead, and Prince of the kings of the earth. Revelation 1:5.

Please note that the firstborn of worship mean the Lord, while the firstborn of the Church mean faith.