What has Christianity truly contributed to humanity

[Neo-Scholastic]

Historian Tom Holland argues that the concept of human dignity and existential equality come out of the Christian tradition.

Even if it did, the Christian establishment of the past centuries wasn't very good at implementing those concepts.  It took the Enlightenment and, to borrow a term from Karl Popper, piecemeal social engineering, to make any inroads.

Your point is well taken. The gap beyween theory and practice persisted for centuries. That said, the Enlightenment did not provide substitute a rational substitute for the religious claim of existential equality; it simply took the Christian ideal for granted, much the way secularists still do. Before Judeo-Christian ideas took hold, the notion of human dignity simply did not exist in pagan cultures. The weak and poor were universally held in contempt. The very notion of a cruxified savior was inconcievable. Nietzsche astutely characterized it as "slave morality" from a classical perspective. The sermon on the mound has no non-Christian parallel in the West., IDK about other parts of the ancient world.