en_tn/2co/10/11.md

What we say

"We" refers to Paul's ministry team. (See: rc://en/ta/man/translate/figs-exclusive)

we say by letter...is the same we as will do when we are there

Paul is stating that he seeks to live by the instructions he writes.

to classify or compare

to classify or compare - "to rank or compare"

have no insight

"show their ignorance" or "show their lack of discernment."