The expression fallen asleep
refers specifically to the death of a believer (see also 1 Corinthians 15:6, 1 Corinthians 15:20, and 1 Corinthians 15:51). As part of his argument, Paul now asks the Corinthians to consider their brothers and sisters who have already died. Those believers died in the hope of being with Christ in heaven. But if Christ had not been raised from the dead, then they have died in their sins (1 Corinthians 15:15) and are lost forever. Again, Paul wishes to highlight the terrible implications of a denial of the resurrection of the body.
18 Then those also who have fallen asleep in Christ have perished.