What does 1 Corinthians 15:18 mean?

Paul is following the logic to show the consequences for the Christian faith if there is no resurrection from the dead, meaning that Christ Himself was not raised from the dead. Paul has shown that if Christ was not raised, the gospel is false. If the gospel is false, then there is no forgiveness for sin by God's grace and through faith in a non-resurrected Christ (Ephesians 2:8–9). The Christian faith is not one of blind, reason-free gullibility. Rather, it's based on a claim of truth, inseparable from the resurrection of Jesus.

Of course, it's logically possible that Christianity could be false, and yet God still be real. So, as Paul notes, debunking the resurrection would not change the reality that sin separates humanity from God (Romans 3:23; 6:23). All those who die with their sin unforgiven by God will be separated from Him for eternity. If Christ was not raised from the dead, then all those who have already died were not saved, after all. Perhaps the Corinthians could think of many specific people they all knew from the Christian community who had died. The idea that all those people remained in their sin and separated from God would have been as much of a blow to them as it is for us to imagine.

Paul will add one more terrible consequence for those who trust in a non-resurrected Christ in the following verse before turning back to show what the reality of the resurrection means for those who believe in what is true.