This verse is a continuation of 1 Corinthians 15:16. The word futile
means to be without purpose,
whereas the word vain
(which is used in 1 Corinthians 15:14) means to be empty.
Without Christ’s resurrection, his death would be to no purpose. In order to save his people from their sin, he had to die and be raised from the dead.
17 Et si Christ n'est pas ressuscité, votre foi est vaine, vous êtes encore dans vos péchés,