Psychological games.
If Christ has not risen, then our faith would be in vain (vs. 14) but of course, that is not the case because Christ has risen from the dead.
You have previously used 1 Corinthians 15:1-4 to teach that some believers will not hold fast to the word/the gospel that Paul preached and lose their salvation, but now you are saying "believed in vain" in verse 4 is connected with "Christ has not risen" in verse 14? So those who believe the gospel are saved, unless Christ has not risen? That's how you now interpret 1 Corinthians 15:1-4?
Paul does not want to give false assurance to those who have believed in vain --
without cause or without effect, to no purpose. Genuine believers hold fast to the word/the gospel that Paul preached and are saved. Those who don't hold fast demonstrate that their belief was never firmly rooted and established.