For if the dead rise not - This is a repetition of what is said in 1-Corinthians 15:13. It is repeated here, evidently, because of its importance. It was a great and momentous truth which would "bear" repetition, that if there was no resurrection, as some held, then it would follow that the Lord Jesus was not raised up.

False witnesses - As having testified the fact of Christ's resurrection, as a matter which ourselves had witnessed, when we knew that we bore testimony to a falsehood. But could five hundred persons agree in this imposition? And if they did, is it possible that some one would not discover the cheat, when he could have no interest in keeping the secret, and might greatly promote his secular interest by making the discovery? Such a case never occurred, and never can occur. The testimony, therefore, concerning the resurrection of Christ, is incontrovertibly true.
If so be that the dead rise not - This clause is wanting in DE, Syriac, some of the Slavonian, and Itala; several also of the primitive fathers omit it. Its great similarity to the following words might be the cause of its omission by some copyists.

(6) For if the dead rise not, then is not Christ raised:
(6) He repeats the same argument taken from an absurdity, purposing to show how faith is in vain if the resurrection of Christ is taken away.

For if the dead rise not, then is not Christ raised. This is a repetition of the argument in 1-Corinthians 15:13 made partly to show the importance of it, and partly to observe other absurdities, following upon the conclusion of it.

The repetition implies the unanswerable force of the argument.