What would one of the biggest impacts on Western Civilization only up to the year 1500?

Question

Serious answers only with sources if you have them.

Answer 1

It would be pretty tough to argue against the answer "Christianity".

Answer 2

Unfortunately, I would have to say gunpowder.

It was discovered very early (142AD) in China, with the first record of its chemical makeup not showing up until around 300AD (also in China)

Gunpowder's influence on Western Civilization really began when Roger Bacon became the first in the world to write about how to make gunpowder in his book "De nullitate magiæ" in 1234 (at Oxford).

In the 14th century, Europe perfected a way to make gunpowder more stable.

After that, bombs, cannons, the flintlock pistol and rifles became inevitable, and by the end of the 1700's, most of the Western World had easy access to cheap well made firearms which would change the world of warfare forever.

http://en.wikipedia.org/wiki/Gunpowder

Answer 3

My guess would be the discovery of the New World (American) by Christopher Columbus in 1492.