Christians are very familiar with the list of sins in 1 Cor. 6. The apostle Paul's famous warning that "the unrighteous will not inherit the kingdom of God" is followed by a clear and stark list of sins that are examples of unrighteousness. There is also in the passage also the great hopefulness that although "such were some of you," through Christ, we have been "washed, sanctified and justified."
But that famous list of sins that "such were some of you" is not the first sin in this text. They are examples of the breadth of unrighteousness people, Christians, have practiced. But the first sin in the text is mistreatment of brethren. That wasn't something that these Christians had just done in the past, it was what a lot of them were currently doing.