1-corinthians 15:14

Then is our preaching vain - Our whole doctrine is useless, nugatory and false.

And your faith is also vain - Your belief of a false doctrine must necessarily be to you unprofitable.

And if Christ is not risen, then is our preaching vain - Another consequence which must follow if it be held that there was no resurrection, and consequently that Christ was not risen. it would be vain and useless to preach. The substance of their preaching was that Christ was raised up; and all their preaching was based on that. If that were not true, the whole system was false, and Christianity was an imposition. The word vain here seems to include the idea of useless, idle, false. It would be "false" to affirm that the Christian system was from heaven; it would be useless to proclaim such a system, since it could save no one.

And your faith is also vain - It is useless to believe. It can be of no advantage. If Christ was not raised, he was an impostor, since he repeatedly declared that he would rise Matthew 16:21; Matthew 18:22-23; Luke 9:22, and since the whole of his religion depended on that. The system could not be true unless Christ had been raised, as he said he would be; and to believe a false system could be of no use to any man. The argument here is one addressed to all their feelings, their hopes, and their belief. It is drawn from all their convictions that the system was true. Were they, could they be prepared to admit a doctrine which involved the consequence that all the evidences which they had that the apostles preached the truth were delusive, and that all the evidences of the truth of Christianity which had affected their minds and won their hearts were false and deceptive? If they were not prepared for this, then it followed that they should not abandon or doubt the doctrine of the resurrection of the dead.

15:14 Then is our preaching - From a commission supposed to be given after the resurrection. Vain - Without any real foundation.