What does 2 Corinthians 9:2 mean?

In the previous verse, Paul noted that he should not need to write anything to the Corinthians about the ministry of a collection for the suffering Jerusalem Christians. Now he says why: He knows—he has confidence—that the Corinthians are ready to give. Paul knows this because he wrote to them in 1 Corinthians about exactly how to prepare for the collection (1 Corinthians 16:1–4). The Corinthians were initially very enthusiastic about participating in the collection.

That's why Paul has been boasting about them to the churches in Macedonia. That includes those in the towns of Philippi, Thessalonica, and Berea. He told those churches that the Corinthians were eager to participate in this gift, and most of those other churches were "stirred up" by the Corinthians' example.

The difference now, though, is that those other churches have already followed through. They have made their contributions while the Corinthians have not yet done so. Paul is ready to see them complete what they were so ready to do a year earlier.