"unless you have believed in vain" (1 Cor 15:2) and salvation by faith

How do proponents of salvation by faith explain this verse: > "Moreover, brethren, I declare unto you the gospel which I preached > unto you, which also ye have received, and wherein ye stand; by which > also ye are saved, if ye keep in memory what I preached unto you, > unless ye have believed in vain." (1 Corinthians 15:1–2, KJV) On the one hand, this passage says that a believer is saved by having believed. However, it also adds a condition: "unless you have believed in vain." That must mean there's a kind of belief—or faith—that is in vain, and therefore doesn’t lead to salvation. What’s especially challenging for the doctrine of salvation by faith, it seems to me, is that this "vain faith" isn't just belief in something minimal, like the historical existence of Jesus, but belief in the gospel itself. So how do proponents of salvation by faith reconcile this verse with their view?