The elements of the Christian cross being both horizontal and vertical exist on a simple flat plane — lines intersecting along specific points on an x-y axis. As symbol it is incomplete, lacking a third dimension, imperfect.
This is not blasphemy or deficient Christology but rather a spiritual observation that human completeness, even that brought by the divine, requires something visible and invisible, an incarnation if you will, and the recognition that even God cannot save without the help of the numb solider who follows human orders obediently — “Crucify him”– and the perfect act of faith obedient to the divine will.
The cross represents the intersection of vertical and the horizontal, heaven and earth respectively. The man on it is intercessor, God and man — the necessary third dimension. A mystery necessary to inject the profane with eternal value. Even violence is tricked into playing into the hands of a glorious act that brings peace.