Why is Jesus called the first fruits of them that slept?
Christ is called the first fruits of them that slept because He was the first person to ever experience true resurrection—God's unique, sovereign act of raising a dead human being not to die again, but to a life of eternity. Lazarus was brought back to life but still had to die, so his case was not resurrection in this sense.
Les teaches that resurrection is not just being brought back to life. Lazarus had to die, illustrating that being restored to physical life is not the same as resurrection. True resurrection is that unique, sovereign act of God whereby He brings back from the dead a human being who has died, not just to die again, but back to a life of eternity. Man's soul or spirit is immortal, though the body is not. Revelation speaks of the resurrection of the just to a life of immortality in God's presence, while the lost are resurrected to a life separated from God, the second death, yet continue through all eternity. Christ was the first person to ever experience resurrection, and that is why He is called the first fruits, plural, of them that slept.