Psalm 73:35-26

Whom have I in heaven (but You)? And besides You, I desire nothing on earth. My flesh and my heart may fail, but God is the rock and strength of my heart and my portion forever. 

Even if our flesh quakes on the outside, our faith comes from the inside. God is a Spirit, and God leads our spirits from the inside. The flesh means nothing when it comes to faith and may react in fear, but our spirits will take over in faith. We are to follow our spirits and not be led by our feelings because feelings change.