It is a falling away, a withdrawal, a defection. Used twice in the New Testament, in the Greek original, to express a total abandonment of the faith.
In this text Paul is addressing those believers who believed the teachings of the Sadducees over our Lord Jesus Christ concerning the resurrection of the dead. The Sadducees taught against any form of the resurrection of the dead. This teaching affected the young church. Paul said that the faith they had placed in Christ for salvation would be in vain or worth nothing if Christ did not raise from the dead. He said that if this life is all we have and that we will not be resurrected to a new life then we are to be pitied.
3. EXERCISED FAITH IN VAIN:
1 Corinthians 15:12-19 ESV (12) Now if Christ is proclaimed as raised from the dead, how can some of you say that there is no resurrection of the dead? (13) But if there is no resurrection of the dead, then not even Christ has been raised. (14) And if Christ has not been raised, then our preaching is in vain and your faith is in vain. (15) We are even found to be misrepresenting God, because we testified about God that he raised Christ, whom he did not raise if it is true that the dead are not raised. (16) For if the dead are not raised, not even Christ has been raised. (17) And if Christ has not been raised, your faith is futile and you are still in your sins. (18) Then those also who have fallen asleep in Christ have perished. (19) If in Christ we have hope in this life only, we are of all people most to be pitied.
The Greek says “faith is in vain” means that the faith has become empty and profitless.