What does 2 Corinthians 2:9 mean?

This refers to the painful letter Paul sent previously to the church in Corinth, sometime after the writing of what we now call 1 Corinthians. The text of that letter is lost to us, meaning God never intended it to be part of the preserved Scriptures. Instead, it served a very specific purpose in this relationship between Paul and the Corinthians and between them and the man who had wronged Paul.

We can see why this was a tricky letter for Paul to write. He had been wronged by a single individual, apparently; that's something he might have normally overlooked. In this case, it seems the offending man was a believer and influential in the church at Corinth. Also, this man's sin seems to have involved challenging Paul's God-given authority as an apostle over the church. In that way, the man was challenging God's authority over the church, as well. Such a sin could not be allowed to stand, especially if any others in the church might follow the man's lead.

Paul says here that he wrote the letter telling the church to correct and discipline this man in order to see if they would be obedient or not. Ultimately, it was a test of their obedience to God's authority, though it was Paul issuing the instructions. The Corinthians passed that test, and now Paul wanted them to pass the next one by forgiving and restoring the repentant sinner.