According to Christian tradition, when St. Peter was crucified, he requested to be crucified upside-down.
He did this because he did not feel worthy to be crucified like Christ. He did not feel worthy to be associated with the symbol of Christ’s sacrifice.
Peter viewed the act of suffering for Christ as a privilege. Many Christians today seem to believe that suffering can and should be avoided. They arrogantly brandish Christ’s symbol as a tribal insignia.
They believe that Christ died so that they could “prosper” materialistically in this life.
They could not be more wrong.
This is why Testing Orthodoxy uses the upside-down cross – St. Peter’s Cross. Peter’s act of humility was Christianity personified.