I write this refers to the letter which he is now writing. Revised English Bible says “In writing this letter.” Whether 2 Corinthians was originally one whole letter, or whether it is made up of parts of two or more originally separate letters, is debated by scholars (see “Translating 2 Corinthians,” page 3), but in any case Paul is referring to the entire letter which he is writing and not just the immediately preceding words (see 10.2, 8).
Be severe: although it is not explicitly stated, clearly Paul is talking about being severe with the Corinthian Christians to whom he is writing. This may be directly stated in translation by saying something like “be severe with you,” or “treat you with force,” or “handle you with a strong hand.”
The authority which the Lord has given: this phrase is almost identical with the one in 10.8. The Lord here and in 10.8 is probably Christ, since Paul claims to be speaking for Christ (12.19; 13.3).
On building up and tearing down, see comments on 10.8, where the same words occur in Greek, and where the object is specifically stated as “you” (the Corinthians). Here it may also be necessary to provide the same object for the two verbs. Contemporary English Version translates “so that I could help you and not destroy you.”
Quoted with permission from Omanson, Roger L. and Ellingworth, Paul. A Handbook on Paul’s Second Letter to the Corinthians. (UBS Handbook Series). New York: UBS, 1993. For this and other handbooks for translators see here .