Introduction and background to the letter.
Paul first addresses factionalism within the church. Paul had planted the church earlier (Acts 18) and guided it for 18 months before moving on. Apollos had then come along and continued to teach there. Apparently, some new Christians had come to believe that there was some conflict between the two. Paul denies that, comparing himself to a foundation builder and Apollos to the one who erects an edifice on the prepared foundation.
Paul uses the metaphor of leaven to condemn immoral behavior within the church. The idea being that if sin is tolerated and excused, it will spread and infect others. He instructs them to expel flagrant sinners from the body. He also chastens them about taking disputes between brothers to the secular authorities. In cases where secular law does not comport with God's law injustice will be the result.
