St. Paul

Thought experiment:

consider a pivotal historical figure;
imagine that he was not born, died young or lived differently;
then imagine how history would have diverged as a result;
this is the premise of an alternative history or time travel story and a potential case for Poul Anderson's Time Patrol. See Who Makes History?

Example:

no Saul of Tarsus;
Christianity remains a small Jewish sect based in Jerusalem, worshiping in the Temple and led by James the brother of Jesus as well as by Peter and other original disciples of Jesus;
this sect ceases when Jerusalem and the Temple are destroyed.

Petrine Christianity was a Jewish sect, worshiping in the Temple and expecting Jesus to return soon to rule the world as Messiah from Jerusalem but that did not happen. When Jerusalem and the Temple were destroyed in 70 AD, most Petrine Christians reverted to orthodox Judaism. That could have been the end of the matter. However, meanwhile, Paul had founded Gentile Christianity.
-copied from here.

My sources on Paul are:

Karen Armstrong, The First Christian: St. Paul's Impact On Christianity (London, 1983);
E.P. Sanders, Paul: A Very Short Introduction (Oxford, 2001).

I will reread these works for any further suggestions towards alternative history.