1 Cor. 15:1-2 (ESV)
Now I would remind you, brothers, of the gospel I preached to you, which you received, in which you stand,  and by which you are being saved, if you hold fast to the word I preached to you— unless you believed in vain.
Paul here returns to the gospel in a penultimate conclusion to his letter. In chapter 16 the doctrinal emphasis of his letter will change to a discourse concerning upcoming practical matters, of which we can still glean plenty of doctrinal instruction.
I am somewhat perplexed by this otherwise adequate translation. Translation is always difficult, especially when you are trying to make use of good English grammar. Verse two is the more problematic. It translates “if you hold fast to the word I preached to you.” But the Greek would more specifically read “and by which you are being saved, in the word I gospelized (preached) to you, if you hold fast to it.” (I like this word gospelized. It puts the importance of preaching not in the law, but in the gospel. Paul doesn’t use the word Keirusw, normally translated preach, but instead he verbs the noun gospel.) Cutting hairs, I know. But I like how Paul emphasizes that the Gospel is a word that saves. We don’t save ourselves by believing it. Faith is a gift from God, through which we hold fast to the word of the Gospel, the word Paul preached to the Corinthians, the word of the death of Christ for our sins, and His resurrection.
“Unless you have believed in vain.” It is possible to lose faith. It is possible to lose sight of the Gospel. This is why Paul wants to reiterate it. Putting the Gospel at the end of his discourse, he allows it to trump, if you will, all the law that has preceded in his doctrinal discourse. Paul has weaved law and gospel throughout the letter. No doubt, by the end of reading the fourteenth chapter, the Corinthian congregation is at the edge of despair for having all their sin exposed. They now see that much of what they were doing was not the good they thought, but rather evil in the sight of the Lord. He wants to comfort them. He will challenge the disbelief of some now, but in doing so he redirects them to the gospel, which is the comfort of Christians. He redirects them to the hope of the resurrection, and justification by faith alone, the article of faith upon which the Church stands or falls. He doesn’t want their faith in the gospel to be lost, to be something in the past. If we have believed in the gospel in the past, but no longer, we have believed in vain. Our past faith cannot save us. This is why we constantly return to church to hear the Gospel, be reinforced, and be strengthened in faith partaking of the Lord’s Supper for the forgiveness of sins. In this way we keep the gospel before us, so that it doesn’t get choked out by the weeds, the toil of this world.