Romans

This is the heavyweight champion of the New Testament. If you want to understand how Christianity works "under the hood," you read Romans. Paul writes this letter to a church he has not visited yet, so he carefully lays out his entire theology. He begins with bad news: everyone, religious or not, is trapped in sin. But then he drops the hammer on the solution: Justification by faith.

He explains that we are made right with God not by following rules, but by trusting in what Jesus did. It is a dense, intellectual masterpiece that covers everything from the sovereignty of God to how to treat your enemies. While Romans deals with high-level theology, Paul also had to deal with messy, real-world church problems, which we see clearly in 1 Corinthians.