In our ordinary walking with God, and in an ordinary course of his dealing with us, the vigour and comfort of our spiritual lives depend much on our mortification…
Every unmortified sin will certainly do two things:—[1.] It will weaken the soul, and deprive it of its vigour. [2.] It will darken the soul, and deprive it of its comfort and peace.
– John Owen –
from The Mortification of Sin in Believers, volume 6 of Works, page 22