But may it never be that I would boast, except in the cross of our Lord Jesus Christ, through which the world has been crucified to me, and I to the world. Galatians 6:14

The significance of the cross of Christ is not found in the thing itself — as in the wood or the shape or even the blood that soaked into it — but in what he brought about by dying upon it. The cross was an object esteemed by the world as representing shame, rejection, and death. So why would Paul find it something to boast in?