Those who sinned before refers to people who had sinned prior to Paul’s second visit to Corinth and who heard his warning during his second visit. In some languages it may be necessary to say explicitly “those who were doing wrong before my last visit.”

All the others refers to any other people who have sinned since his second visit, either by immoral behavior or by supporting the “false apostles.” The warning is that Paul “will not be lenient” (New American Bible) with offenders when he comes for the third time.

Some manuscripts add the words “I write” after the words “being absent now” in Greek, and King James Version and Reina-Valera revisada follow this reading: “and being absent now I write to them.” Though the original Greek almost certainly did not include the word translated “I write,” the addition does not change the meaning.

That if I come again I will not spare them: the word that may introduce an indirect quotation as in Revised Standard Version, or it may be the equivalent of quotation marks: “I said, ‘The next time I come, nobody will escape punishment’ ” (similarly Good News Translation, Bible en français courant, New International Version).

If I come again: the Greek probably means nothing more than “when I come again” (see Good News Translation, Bible en français courant, New International Version), and more than likely it does not mean that Paul had doubt about whether he would visit the Corinthian church again.

I will not spare them: Paul will not hesitate to exercise strong discipline against the offenders. The verb translated spare is the same as in 1.23 as well as 12.6, where it is translated “refrain” in Revised Standard Version. Here the idea is to refrain from punishing the guilty persons, although there is no expressed object in Greek. The pronoun them has been added by Revised Standard Version. But Translator’s New Testament adds the pronoun “you.” New Revised Standard Version revises to say “I will not be lenient,” while New Jerusalem Bible reads “I shall have no mercy.”

The structure of this verse may have to be simplified in certain languages. The first verb indicates something that Paul did on his previous visit, his second to Corinth. Translators may change the order of the statements as follows:
• On my second visit with you I warned those who did wrong and everyone else to stop doing wrong. Now that I am far away from you, I am giving you the same warning. When I come again, no one who does wrong will be able to avoid discipline.

Another possible model, putting the quotation first, may read as follows:
• This is what I say: “When I come again, no one who does wrong will escape discipline.” On my second visit to you I warned those who did wrong and all others to stop acting that way. Now that I am far from you, I am giving the same warning again.

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 .