In this book, Paul puts on his "war paint" and goes after those who insisted Christians needed to adopt some practices from the Old Testament like circumcision.
1 Cor 5:11 - but as it is, I wrote unto you not to keep company, if any man that is named a brother be a fornicator, or covetous, or an idolater, or a reviler, or a drunkard, or an extortioner; with such a one no, not to eat.