The most central and material part:
core, essence, gist, heart, kernel, marrow, meat, nub, pith, quintessence, root¹, spirit, stuff, substance.

Deep calls to deep in the roar of your waterfalls;
all your waves and breakers have swept over me.
By day the LORD directs his love, at night his song is with me, a prayer to
the God of my life.

Psalm 42:7-8