1 Corinthians 15:13
But if there be no resurrection of the dead, then is Christ not risen:
Note 3 at 1Co 15:13: Paul tied our future physical resurrection together with the resurrection of Jesus in an inseparable way (this verse and 1Co 15:16). He was saying that anyone who does not believe in the bodily resurrection of all mankind does not believe in the bodily resurrection of Jesus. There can be no salvation without believing in the resurrection of Jesus (Ro 10:9), so a person who does not believe that everyone will be physically resurrected cannot be a Christian.
This put the Corinthians who had believed this lie in an awkward place. They either had to reject the lie about no resurrection or they had to reject Jesus. They could not have it both ways. This still holds true today.