This verse expands and illustrates the meaning of verse 3, but does not do it logically. So Revised Standard Version‘s For is a weak connection. It may be omitted in translation (so New International Version, Revised English Bible).

I belong can be rendered in many languages as “I am a disciple of.” See also the comments on 1.12.

The last part of the verse needs expansion in some languages, including English. The literal answer to the question are you not merely men? is clearly “Yes, we are.” But verse 3 has shown that the question is not intended literally; it is a rhetorical question. It describes a purely human way of life lived apart from the Holy Spirit. For this reason Fee renders this clause as “… are you not acting like mere men?” One may also translate this clause as a statement; for example, “you are acting like worldly people!” or “you are acting according to human standards!”

Quoted with permission from Ellingworth, Paul and Hatton, Howard A. A Handbook on Paul’s First Letter to the Corinthians, 2nd edition. (UBS Handbook Series). New York: UBS, 1985/1994. For this and other handbooks for translators see here .