The cross ... this object, IMO, and with a historical understanding of Jesus of Nazareth in tow, represents the highest challenge to humanity to not only embrace the "Kingdom (realm) of God" which hung Jesus there, but also the lengths which its (the Kingdom) challenging importance beckon.
In other words, the cross was the culmination of Jesus ideological dedication and practical social implementation of the tangible characteristics of the "Kingdom." These are the very things that resulted in his death by the cross. While most dedicated and well meaning Christians see the cross as a divine rescue from their sins and earth pangs, I see it as an incredible symbolic representation of the incredible challenge contained in an authentic existence within this very social and universal "Kingdom" which was and remains grounded in ideas concerning fairness, equality, human rights, activism, and open tables. There is, IMHO, nothing which induces a personal sense of security - in the now or in an eternity somewhere - existing in any of these "Kingdom" initiatives.