In Paul’s writings salvation is often depicted as an ongoing act of God in the life of a believer. Through his Holy Spirit he enables the believer to apply the gospel to all of life. That is what being saved
(note the present tense of the verb) means.
It is through faith in the gospel that we first receive salvation. The same is true of our ongoing salvation (also called our sanctification,
Romans 6:22). God always saves by
the gospel.
Furthermore, Paul gives a warning similar to that in Hebrews 4, where believers are warned not to follow the example of those whose faith turned out to be false and short lived. That is what it means to have believed in vain.
Paul has just stated, however, that the Corinthians have received
the gospel and that they stand
in it (1 Corinthians 15:1). Despite his warning he trusts that they will hold fast to the word
he has preached to them.
Although Paul will only make known the cause of his concern in 1 Corinthians 15:12, these words should already indicate to the Corinthians that he has something serious to discuss with them.
2 and by which you are being saved, if you hold fast to the word I preached to you unless you believed in vain.