John Calvin Commentary 1 Corinthians 15:2

"by which also ye are saved, if ye hold fast the word which I preached unto you, except ye believed in vain." — 1 Corinthians 15:2 (ASV)

If you keep in memory — unless in vain. These two expressions are very cutting. In the first, he reproves their carelessness or fickleness, because such a sudden fall was evidence that they had never understood what had been delivered to them, or that their knowledge of it had been loose and floating, since it had vanished so quickly. By the second, he warns them that they had needlessly and uselessly professed allegiance to Christ if they did not hold fast this main doctrine.