I can't tell if you're bringing this up because you're reading and studying the chapter and had a question, or if you saw this verse that someone cherry-picked and you immediately came here to ask about it instead of studying it first? Your post seems to indicate the latter, and thus, my answer is to go back to the chapter and read is slowly, making sure you understand it all before proceeding.
But if there be no resurrection of the dead, then is Christ not risen: And if Christ be not risen, then is our preaching vain, and your faith is also vain. Yea, and we are found false witnesses of God; because we have testified of God that he raised up Christ: whom he raised not up, if so be that the dead rise not.
-1Co 15:13-15
So what is the subject matter here? He is addressing those who would teach that Christ did not rise from the dead, or that there was no man who was raised from the dead by Christ.
If we cherry-pick verse 15 in willful ignorance, then we can conclude that the apostles are false witnesses and false teachers. That's absurd, and that's taken out of context. He's saying IF Christ was not raised from the dead, then we would all be false witnesses, but Christ IS raised from the dead, therefore, the apostles are true witnesses, and we speak the truth in Christ.
Else what shall they do which are baptized for the dead, if the dead rise not at all? why are they then baptized for the dead?
-1Co 15:29
Thus, IF Christ was not raised from the dead, then why are people baptized at all? Why be baptized for the dead? That would make no sense. However, because Christ IS raised from the dead, therefore, the doctrine of baptism is sure.
It seems no one raises a point of contention on the next verse, but Paul continues this line of questioning in the next verse:
And why stand we in jeopardy every hour?
-1Co 15:30
Again, IF Christ was not raised from the dead, why do the apostles put their lives at risk every hour of every day? It would not make sense to do so. However, because Christ IS raised from the dead, therefore, their jeopardy [exposure to hazard, death, and peril] is not only logical, but sound in the faith.
In short, he's asking these questions to test the reasoning processes of those who are reading, and that those who have faith in Christ should not question the resurrection of the dead, but believe soundly on Him.
