1 Corinthians 15:15

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 (KJV)

Then: Corinthians now hear a third point(Baker, p. 219). If Christ did not rise from the dead, then all those five hundred plus witnesses, the disciples, the apostles and Paul himself are liars. Even worse, they are swearing to God that what they said was true. How could that many people say the same thing and none state otherwise? How could so many risk their fate with the God of heaven? It couldn't be. They didn't lie. Christ rose.

Now: No one really takes time to say that the resurrection didn't happen. Usually atheists just deny the existence of God. They never get to the point of considering resurrection. But those who believe concentrate on the resurrection, because that event is the "word of the cross" (Thiselton quotes a discussion by Harrisville, p. 1171).

Extra Source: Cornerstone Biblical Commentary 1 Corinthians (William Baker), 2 Corinthians (Ralph Martin & Carl Toney), ed. by Philip Comfort. Carol Stream: Tyndale House Publishers, c2006.
The First Epistle to the Corinthians: a commentary on the Greek text by Anthony C. Thiselton (Authentic Media, c2000) online at http://books.google.com/books?id=IHG_DNLpmroC&dq=1st+Corinthian+commentaries&printsec=frontcover&source=in&hl=en&ei=W9fQSpC1GILplAfHiumoCg&sa=X&oi=book_result&ct=result&resnum=11&ved=0CB8Q6AEwCg#v=onepage&q=&f=false