As Christ warned repeatedly that unrepentant people will die in their sins, it is easy to guess who are they. We see that pride resists love-God and derivative repentance. For that Satan, the spirit of pride, fell from heaven of life running towards soul suicide.
Some may ask “when souls are eternal how they can die?”
Our souls (personalities) are made of spiritual organs made of spiritual atoms bonded together with spiritual love. When we fell from heaven following pride-devil our internal love-bond began to diminish as pride is enemy to love. Only culture of repentance could turn us to love, which would have strengthened the internal bond. But proud we resisted repentance and continued to culture pride. It finally dissolves that bond here or after death. Then falling apart we lose soul integrity i.e. personality as we see in stark mad people here. Though soul dies, the degenerate soul parts remain to suffer eternally from their likes.
Those, who still retaining personality but resisting repentance, is potentially dead as they are sure to lose it in future. So these worldly people are called dead by Christ.