"Now I would remind you, brethren, the gospel I proclaimed to you, which you received, in which you stand, through also which you are being saved, if you hold on to it fast. Otherwise you believed in vain." (1 Corinthians 15:1-2).
What does it mean to have believed in vain? Why is Paul exhorting his Christian brothers to hold on to the gospel which was proclaimed to them?
Perhaps some Calvinist would like to pretend that Paul is saying that if some do not hold on to the gospel they did not have a "saving faith" and were never saved in the first place. This is quite a common ploy among Calvinists who seek to avoid the teachings of the Scriptures. But will it work for them here? Not a chance.
Paul is writing to the Corinthian church whom he calls under inspiration of the Holy Spirit, "those sanctified in Christ Jesus." Not only so, Paul is exhorting his "brethren to hold on to the word he preached to them. He tells them they are "being saved" if indeed they hold on to their faith. You cannot hold on to something you do not have. It is rather obvious that he is talking to people who are Christians. Now if Paul is assuming these people are Christians, what sense does it make to tell them they may have believed in vain if in fact they do not hold on to the gospel which he preached to them? It would not make any sense at all. Such a ploy would only seek to nullify the word of God for the sake of one's traditions.
Paul is exhorting his brethren in Christ who are being saved to hold on to the faith or these brethren in Christ have believed in vain. He ends this very discourse with the same message by saying, "Therefore, my beloved brethren, be steadfast, immovable, always abounding in the work of the Lord, knowing that in the Lord your labor is not in vain." (15:58). Can we not trust the word of God and know that he is warning his Christian brethren they will have believed in vain if they do not hold fast to the gospel? I think we can.
You can only hold on to something you already have. And you can only say one is "being saved" if they are a Christian. The word "vain" here in the original Greek suggests the idea of failure or falling short of success. He tells (a) his brethren to (b) hold on to what they already have and if they do not hold on to it (c) they will have faith they have had since Paul preached to them will have been in vain. We do not need to think long to realize this is Paul's message to them if we are honest with ourselves and the word of God.