Paul calls Christians to catechize and root the new disciples in faith and in practice, but he focuses on three issues in chapter 4 under the subject of holiness: sexual issues, love of the brotherhood, and those Christians who have died. For Christians, it is vitally important that they do not give those who would oppose them an excuse that a sexual scandal might afford, and Paul uses language to describe this that may be shocking to modern readers.
God wishes us to become sanctified, which is a process that continues through our lives. Heb 12:14 commands us to be holy, else we will not see God. Sexuality is a mutual gift for giving and receiving by married couples, and this necessarily means that Christians cannot be unchaste, and so Paul calls Christians to control themselves sexually, and God will avenge the wrongs that have been done against others sexually.