Before there was even an organized "Christianity," the crucifiction and resurrection of Jesus had the biggest impact on the Christian generations to come.
Several events occur to me as having a huge impact on Christian believers.
The conversion of Constantine, his establishment of the Eastern Orthodox church, and as the first Christian Roman Emperor
The compilation of the sacred books of the Bible
The spread of Islam
The Protestant Reformation
Which event do you think had the biggest impact?
I would add these two:
AD37 Conversion of Paul
AD 1440 Invention of the movable type printing press by Gutenberg.
posted by bobpursley
I think the first one because without it,
Christianity, it wouldn't even begin or be well-known as of today.posted by Gretchen
Some Christians might argue with you. They feel that Christ's message is far more important than his supposed resurrection.
However, your answer is certainly one with many Christians would agree.