1 Corinthians 15:1-2

Paul is now reminding those at the Corinthian church about the gospel. They have received the gospel and now stand in it (v1).

and by which you are being saved, if you hold fast to the word I preached to you-unless you believed in vain.

(v2, ESV)

So, they are being saved by the gospel if they hold fast to Paul's preaching. However, this doesn't happen if they believed in vain. This brings upon a scary thought that one can believe in vain. The words of Jesus come from Matthew.

Many will say to me on that day, 'Lord, Lord, did we not prophesy in your name, and in your name drive out demons and perform many miracles?' Then I will tell them plainly, 'I never knew you. Away from me, you evildoers!'

(Matthew 7:22-23, NIV)

It is quite the scary thought. Though we aren't saved by works, we should receive, stand firm and hold fast to the word preached. "unless we believed in vain" (v2, ESV). Let us not believe in vain.