33 A.D.
Crucifixion of Christ
-I would stop it from ever happening
It would save a lot of lives over the course of history
I know you are infamous for your mathematics prowess, but have you read a single history book?
Read up on the Sino/Japanese wars or the way African and Native American tribes treated each other or how the Mongols treated their victims or what Pol Pot did to Cambodia or what Mao's great leap forward did. All of those atrocities were completely absent any Christian influence.
We'd also likely still have widespread slavery, fewer women's rights and more of a caste system without Christianity.
All of Western Civilization's morality and standards were developed under the umbrella of Christianity. The very concepts of what you believe is good or right was directly shaped by Christianity.
And before you claim Jesus' teachings were all we need, without the grace and forgiveness that comes through Christ's death, burial and resurrection, there would be no Christianity and none of what he taught would have survived a decade, much less 2000 years.