Albert Barnes on 1 Corinthians 15:13

But if there be no resurrection of the dead. If the whole subject is held to be impossible and absurd, then it must follow that Christ is not risen, since there were the same difficulties in the way of raising him up which will exist in any case. He was dead; and was buried. He had lain in the grave three days. His human soul had left the body. His frame had become cold and stiff. The blood had ceased to circulate, and the lungs to heave. In his case there was the same difficulty in raising him up to life that there is in any other; and if it is held to be impossible and absurd that the dead should rise, then it must follow that Christ has not been raised. This is the first consequence which Paul states as resulting from the denial of this doctrine, and this is inevitable. Paul thus shows them that the denial of the doctrine, or the maintaining the general proposition, "that the dead would not rise," led also to the denial of the fact that the Lord Jesus had risen, and, consequently, to the denial of Christianity altogether, and the annihilation of all their hopes. There was, moreover, such a close connexion between Christ and his people, that the resurrection of the Lord Jesus made their resurrection certain. See 1Thes 4:14. Jn 14:19.

(c) "but if there be no resurrection" 1Thes 4:14