For the most part, 2 Corinthians is one long defense of Paul's apostleship. Paul's relationship with the church at Corinth was apparently a rocky one. Between the writing of 1 and 2 Corinthians, Paul made an unplanned visit to Corinth which turned out to be painful for him. After this painful visit, Paul wrote another letter to the Corinthians. Since this letter is now lost, it is impossible to determine its exact content but Paul says that he wrote it "out of much affliction and anguish of heart and with many tears." In addition to this painful visit and letter, Paul also has to account for a promised trip to Corinth which he never made. As we read the beginning of 2 Corinthians, we get the sense that the Corinthians are clamoring for an explanation of Paul's seemingly unreliable actions...and if Paul is unable to give such an explanation then he will likely lose all credibility with the Corinthians as their apostle and pastor.
Paul answers the Corinthians' concern in a number of ways in the first 5 chapters of 2 Corinthians but there is one resounding theme throughout those chapters: the Corinthians are judging Paul's apostleship by the wrong standards. The Corinthians question the trustworthiness of Paul's word because he changed his travel plans so many times but Paul's says in chapter 1 that his word is trustworthy not because his itinerary never changes but because the word he preached transformed the Corinthians themselves. The Corinthians question Paul's authority because he didn't come with letters of recommendation (apparently other missionaries to Corinth did) but in chapter 3 Paul says that the Corinthians themselves are Paul's letter of recommendation. This train of thought comes to a climax in chapter 5 where Paul says that the "love of Christ controls us," not an itinerary or official signs of authority. The love of Christ is the one and only thing that determines Paul's actions. And Paul says that the love of Christ can be summed up this way:
"and he died for all; therefore all have died; and he died for all that those who live might no longer live for themselves but for him who for their sake died and was raised. From now on, therefore, we regard no one according to the flesh. Even though we once regarded Christ according to the flesh, we regard him thus no longer. Therefore, if anyone is in Christ - new creation!"
The Corinthians keep looking for signs of Paul's authority and apostleship but Paul is saying to them "Look no further than yourselves! I preached the death and resurrection of Christ to you and that message made you new creatures in Christ! What more proof do you need?" Paul's changed travel plans and letters of recommendation are small beans compared to the miraculous work of the Spirit that has already taken place among the Corinthians. It is God's new creation already taking root among them which is the one true sign that Paul speaks for God as he has claimed.
It is sooooooooo tempting for us to look for any other way to validate our ministry. We find countless ways to measure and quantify; to provide letters of recommendation to prove to others that our ministry is the real thing. But there is really only question that we should be asking: Is the love of Christ leading to new creation among us?