1 Corinthians 15:14

And if Christ be not risen, then is our preaching vain, and your faith is also vain (KJV)

Then: Vain equals empty, useless, not worth anything. Baker (p. 217) marks the meaning as "without results." This is Paul's second point. If Christ's resurrection did not happen, then there is no reason for anyone to continue preaching about the Lord's life, his teachings, and his being. There is nothing for anyone to believe in. An absolutely dreary supposition. No joy, no life.

Now: For sure Christians know that Christ was raised from the dead, after sacrificing his life so that they would be saved from eternal judgement and condemnation. Through Christ they are forgiven. Without that, there is no purpose or reason for anyone to follow what at best would be a "good man." It would also be a denial of the Holy Spirit within them.

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.