A.T. Robertson Commentary 1 Corinthians 15:14

"and if Christ hath not been raised, then is our preaching vain, your faith also is vain." — 1 Corinthians 15:14 (ASV)

Vain (κενον). Inanis, Vulgate. Old word, empty. Both Paul's preaching and their faith are empty if Christ has not been raised. If the sceptics refuse to believe the fact of Christ's resurrection, they have nothing to stand on.