He is using 2 Corinthians 10:6ff to really drive home the point that we must be ready to argue for the faith. From the Department of ?Let?s Just Love Jesus and Ignore Theology? Comes This? | Timothy J. Hammons How do native speakers read 10:6ff?
I understand it to mean (and if I had to read it aloud I would say) 'the Second Epistle to the Corinthians, chapter 10 beginning at verse 6'.
'6ff' means 'verse 6 and (an unspecified number of) verses following.