'Yes, we are found false witnesses of God, because we witnessed of God that he raised up Christ, whom he did not raise up, if so be that the dead are not raised.'
Now we come to see why Paul went into such detail as to the witnesses of the resurrection. With Peter, James the Lord's brother, the twelve, all the Apostles and the covenant community all involved as witnesses the Corinthians will certainly be disassociating themselves from the whole church if they deny their reliability. In order to deny the resurrection they will have to declare them all false witnesses. That is, they will have to declare the accepted revealers of God and those who had walked with Christ on earth to be false witnesses about God.
For they had all seen Christ when He had been raised from the dead, and bore witness of God that He had raised Him up. Yet if we dogmatically say that the dead do not rise then Christ cannot have risen. For if He has then at least some dead do rise. Thus either the Apostles and their fellow-believers are liars and false witnesses about God or it is possible for the dead to rise.