Final answer:
Preaching is in vain if Christ has not risen, as the resurrection is a fundamental belief validating Christian teachings.
Step-by-step explanation:
If Christ be not risen, then preaching is also in vain. This question pertains to one of the key beliefs in Christianity, that Jesus Christ's resurrection is the cornerstone of the faith. According to the New Testament and Christian theology, the resurrection is a pivotal event that validates Christ's divinity and the truth of his teachings.
If the resurrection did not happen, then all the efforts and teachings of Jesus and His apostles, such as preaching the good news and the promise of salvation, would be without foundation or purpose. Hence, preaching is the act that would be considered in vain without the resurrection, as it is deeply intertwined with the message of Christ's victory over death and his role as the savior.