The cross is foolishness. Of all the aspects of the gospel message to pick as the summary word the cross is nuts. It's weak and unimpressive. Yet, Paul summarises his message as "Christ Crucified" but we know he says all the other stuff too about God's promises to Abraham, about him being king over all, about us inheriting all things with him, about wrath and resurrection etc.
The cross makes an idiot of the world who thought they'd beaten Jesus by the cross. It also stops us being proud because no one looks big when they say that Jesus on a tree is where our hope comes from. The cross is the Bible's gospel-code. When we talk about being cross-centred we mean being about the gospel of salvation history from Genesis to Revelation.
Likewise look at Esther. Purim is the festival to celebrate the victory over the Amalekites. You'd think GALLOWS would be the symbol cos they hang their enemy and his sons on Gallows (Haman the Amalekite from the family of Agag). What do they pick as the symbol - PUR - because their enemy thought that by casting lots (pur) and trusting in chance he could beat the Jews. The symbol mocks the enemy. Luckyman lost and it's celebrated by building Vegas. It's comedy as God tramples his enemies under foot without being mentioned.
You'd think that an empty tomb or a throne would be the best symbol of Christianity but that's a power statement. The cross makes a fool of the watching world but it's everything to us. No one looks clever when they they sing of the wondrous cross where Jesus was cursed to bring us blessing. There is comedy as God mocks the world and wins the victory.
See also: Watch Mark Driscoll making some similar points about humour/satire in the Bible, Read a outline of main points of Mark Driscoll on Humour by Brian Lair.