They are unable to comprehend, nor to truly perceive that spiritual truth is true or real. They remain in total spiritual darkness - unable to respond to spiritual stimuli, as a physically dead person can't respond to
physical stimuli. They are as dead spiritually as a physically dead person is physically dead.
[
1Co 2:14 KJV] 14 But the natural man
receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know [them], because they are
spiritually discerned.
[
Jhn 1:4-5 KJV]
4 In him was life; and the life was the light of men.
5 And the light shineth in darkness; and the
darkness comprehended it not.