Vain (mataia). Old word from adverb matēn (Mat 15:9), devoid of truth, a lie. Stronger word than kenon in 1Co 15:14.
Ye are yet in your sins (eti este en tais hamartiais humōn). Because the death of Christ has no atoning value if he did not rise from the dead. In that case he was only a man like other men and did not die for our sins (1Co 15:3).