{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.

Jamieson-Fausset-Brown Bible Commentary

16. The repetition implies the unanswerable force of the argument.

Matthew Henry's Concise Commentary

15:12-19 Having shown that Christ was risen, the apostle answers those who said there would be no resurrection. There had been no justification, or salvation, if Christ had not risen. And must not faith in Christ be vain, and of no use, if he is still among the dead? The proof of the resurrection of the body is the resurrection of our Lord. Even those who died in the faith, had perished in their sins, if Christ had not risen. All who believe in Christ, have hope in him, as a Redeemer; hope for redemption and salvation by him; but if there is no resurrection, or future recompence, their hope in him can only be as to this life. And they must be in a worse condition than the rest of mankind, especially at the time, and under the circumstances, in which the apostles wrote; for then Christians were hated and persecuted by all men. But it is not so; they, of all men, enjoy solid comforts amidst all their difficulties and trials, even in the times of the sharpest persecution.