1 Corinthians 15:14 (ESV)

14 And if Christ has not been raised, then our preaching is in vain and your faith is in vain.

If Christ has not been raised, it means that his redemptive work was unsuccessful. In that case, believers are still in their sins and death still reigns over them. This will make the gospel, both its preaching and the faith of those who hear and accept it, vain (or empty).