Can you explain to a Buddhist the role of the Crucifixion in the Church before the 10th century?

Please allow me to draw an analogy from Buddhism as an illustration: Your question would be similar to a Catholic asking what role Siddhartha Gautama’s (ca. 563 B.C.-483 B.C.) enlightenment under the bodhi tree had on Buddhism prior to the fifth century A.D., approximately 10 centuries later. Just as the Buddha’s enlightenment is foundational to understanding the history of Buddhism from its inception on, so too is the Crucifixion to Catholicism. In other words, around their foundational events both Buddhism and Christianity revolve throughout the whole of their history.