I preached through 1Cor. 15 very recently. I think this about the most difficult verse in the epistle to understand. After a good deal of study, I came away with the feeling that he may be referring to people who were saved on account of the testimonies of Christians who had since died - perhaps especially because they faced death with confidence and courage and hope. In other words, the testimony of these Christians, no longer living, was instrumental in the conversion of those who were being "baptized" here. The argument, then, would be that the hope with which these Christians faced death would be an empty hope, were there no resurrection of the dead, and, accordingly, those who were converted/baptized because of that hope were miserable deluded. They were going to be ready to give up there earthly lives for no good reason.
