Philippians 2:8
8 And being found in appearance as a man, He humbled Himself and became obedient to the point of death, even the death of the cross.
The cross is the symbol of Christianity. It symbolises the crucifixion of our Lord and Saviour. It represents the suffering, public humiliation, the shaming and the passion of our Lord Jesus Christ.
Paul boast was in the Cross of Jesus, Galatians 6:14
14 But God forbid that I should boast except in the cross of our Lord Jesus Christ, by whom the world has been crucified to me, and I to the world.
Paul was not ashamed of the Gospel neither was he ashamed of the cross of Jesus.
Over and over again Jesus calls His disciples to deny themselves, pick up their cross and follow Him. Jesus was clear about His followership, who ever wanted to follow Him had to deny self and pick up the cross and follow Jesus. The way of discipleship is the way of the cross.