Some of the Corinthians were professing unbelief in the prospect of a general resurrection. Paul runs the argument backwards. He says the repudiation of the resurrection at the last day necessarily involves the repudiation of God's resurrection of Jesus. This would have the further implication of denying the atonement. Forgiveness is like a dud check which cannot be cashed - if there is no Easter. Justification by faith is empty if there is no dimension of hope beyond the grave.
I'm busy working on my blog posts. Watch this space!