'Now if Christ is preached that he has been raised from the dead, how say some among you that there is no resurrection of the dead? But if there is no resurrection of the dead, neither has Christ been raised.'
Some of the Corinthians were declaring that there was no resurrection of the body from among the dead, as though such a thing could not be. Paul counters by pointing out that Christ has risen from the dead in bodily form, and is still alive (perfect tense - has been raised and is still raised). Thus their basic premise is wrong. Then he reverses the argument. If there is no resurrection from the dead then Christ has not been raised, and therefore the Gospel they preach as defined in 1 Corinthians 15:2-4 is a vanity which is not true, and the witnesses, Peter, James the Lord's brother, all the Apostles and the covenant community, are all found to be liars. Furthermore as their own faith is dependent on that Gospel they too are still in their sins because their belief too is false and useless, and those who have died in Christ are not merely sleeping but have perished.
'But if there is no resurrection of the dead, neither has Christ been raised.' Some see this as simply saying that if, dogmatically speaking, there can be no resurrection from the dead, Christ cannot have been raised. Others take it further and add that Paul means that the resurrection of the dead as a coming reality and the resurrection of Christ go together. If one occurred then the other will occur. Thus if Christ has risen, the resurrection of the dead is guaranteed.