Charles Ellicott 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 ye keep in memory what I preached unto you.—Better, if ye hold fast with what word I preached the gospel to you, unless you believed in vain. The idea here is not, as implied in the English version, that they were converted, and yet that heretofore no results have followed from their belief; it is the same thought which comes out more fully in 1 Corinthians 15:17. They are saved by their faith in the gospel as preached by St. Paul, unless (which is impossible) the whole gospel be false, and so their faith in it be vain and useless.