1 Corinthians 15:2

By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain (KJV)

Then: What does Paul mean "believed in vain?" This is the last of nine times that Paul uses the word "save" in this letter (Baker, p. 210). Its present passive tense gives a present/future meaning to the word. Vain? Back up. Maybe their previous actions indicated lack of trust or unbelief in Christ, but no one ever wants that to be the case. Paul never accused them of not loving Christ, but the question lies here dormant. What's the way out? Hold the truths of the gospel in their minds. Especially rely on Christ's resurrection for that is the validation of all the Lord's teachings (Henry).

Now: Believers hold the salvation gospel in common. It starts with belief, salvation and baptism. It expects continued, everyday acts of faithful obedience. This becomes a type of future salvation, and if a person's not careful, it can jeopardize ultimate relationships with Christ (Baker, p. 212).

Extra Source: Cornerstone Biblical Commentary 1 Corinthians (William Baker), 2 Corinthians (Ralph Martin & Carl Toney), ed. by Philip Comfort. Carol Stream: Tyndale House Publishers, c2006.
Commentary on the Whole Bible Acts to Revelation Vol VI by Matthew Henry online at http://www.ccel.org/ccel/henry/mhc6.iCor.xvi.html