4 “Yea, though I walk in the midst of the shadow of death”. Yea, though I walk in the midst of this life, which is the shadow of death. “I will fear no evil, for You are with me.” I will fear no evil, for You dwell in my heart by faith: and You are now with me, that after the shadow of death I too may be with You. “Your rod and Your staff, they have comforted me.” Your discipline, like a rod for a flock of sheep, and like a staff for children of some size, and growing out of the natural into spiritual life, they have not been grievous to me; rather have they comforted me: because You are mindful of me.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)