The Roman cross — instrument of execution and, in Christian theology, the pivot of history. Paul boasts in nothing except the cross (Gal 6:14) and determines to know nothing but Christ crucified (1 Cor 2:2).
The cross was a symbol of shame and defeat in Roman culture. Paul's theology radically inverts this: the cross is the power of God (1 Cor 1:18). The stumbling block to Jews and folly to Greeks is God's wisdom and power.
Colossians 1:20 says God reconciles 'all things' through the blood of the cross — cosmic reconciliation through a Roman execution device. This is the central scandal and glory of the Christian message.