1 Corinthians 15:17,18

“Now, if Christ be not raised, your faith is vain; ye are yet in your sins. 18. Then they also which are fallen asleep in Christ are perished.”

Once deny Christ's resurrection, and there is no more salvation in Him.

The word ματαία denotes, as often, the vanity of the thing from the standpoint of its effects, its uselessness. Such is the difference between it and the κενή, vain, of 1 Corinthians 15:14. Faith in the resurrection, not taking hold of a real fact (κενή), cannot procure for the believer the salvation he expects (ματαία). It is completely to mistake the meaning of this saying, to follow Heinrici and several others, in applying the expression: to be yet in one's sins, to the moral bondage of sin. The apostle certainly does not mean: “If Christ be not really risen, you will not be able to conquer your evil inclinations.” Nothing in this Epistle has prepared us for such an idea. It is of the state of condemnation arising from unpardoned sins that he wishes to speak, as is clearly shown by the following verse. The idea is this: Condemnation can only be taken away by the expiatory death of Christ, and expiation would never have taken place if the victim who accomplished it had not been restored to life. As long as the security is not let out of prison, it must be concluded that the debt is not paid. If then Christ did not leave the prison of death, our justification was not obtained by His death; and we are still, we believers, as much as others, condemned. Bonnet rightly says: “No one can understand the doctrine of Scripture regarding the resurrection, unless he has clearly present to his mind the intimate and indissoluble relation there is between sin and death.” Christ dead without resurrection would be a condemned, not a justified, Christ. How could He justify others?

Hence there follows immediately the disastrous consequence drawn in 1 Corinthians 15:18: the perdition of those who have been seen to die peacefully in the faith of Christ.