Paul closes out his letter by somewhat threatening the Corinthians, saying he's going to come and be harsh with them if they don't get some of their sin issues dealt with. He also invites them to test themselves to see if they are truly in the faith. 1 John helps us with that test.
Paul has a situation in Corinth where a man had sinned against someone in the church. He lets them know the sin isn't just against him, but against everyone. He begged them to deal with the situation so the man could be restored. He also encouraged them to forgive him, so that Satan didn't get a foothold in the church.