1 Corinthians 5:9 - Matthew Poole's English Annotations on the Holy Bible

It should seem that Paul had wrote so in some former epistle which he had directed to this church, which is lost; for we must think that Paul wrote more epistles to the several churches than those left us upon record in holy writ (yet so as not to undermine the perfection of the Holy Scriptures). By fornicators are meant any sorts of unclean persons known to them; and the keeping company with them, which the apostle had prohibited to the Corinthians, was not a mere fellowship with them in their works of darkness, but any intimacy of communion with any such persons.