In 1 Corinthians 5:9–13 Paul clears up a misunderstanding that is directly related to the problem discussed in 1 Corinthians 5:1–8. It appears that, in a previous letter, he had already warned the Corinthians not to associate with sexually immoral people
(or with people who committed other gross sins; see 1 Corinthians 5:10–11).
9 I wrote to you in my letter not to associate with sexually immoral people