Neither the church nor Christianity itself sprung up fully formed. That's what you walk away from the book with--you knew it before but Davidson does a good job of filling in the details. After Jesus' death, what happened? What did it mean to believe? Did one have to change one's behavior? If worship was part of it, where did it happen (and this evolved over time). Every detail led sometimes to agreement, compromise, argument, even war and sometimes splits in the church.
One of the things I found most interesting (especially since I'd never thought about it before) is how the Christians became split off from the Jews. Originally, they were both seen as and considered themselves as Jewish. What changed this is when Gentiles began to be converted and not just Jews. What did it mean to be a Christian? Did one have to follow all of the Jewish laws that were not contradicted by what happened during and after Christ's lifetime? Finally, there was circumcision. Unsurprisingly, there was not a big desire to undergo it on the part of converts. When the decision was made not to require it, that is when the Christian church began its separation from Judaism.