Paul's Second Letter to the Corinthians is a strange and wonderful piece of Biblical literature. A significant aspect of its strangeness is explained by the scholarly consensus that the letter is composed of selections from at least three separate letters sent by Paul. Thus the abrupt changes in tone and content arise not from Paul's moodiness or inability to stay on topic, but from the differing circumstances of the different sections. The other factor in the epistle's strangeness is the deeply personal nature of these fragments – they reveal less about what the controversies are in Corinth than about how those controversies have pained Paul personally.
Though the exact circumstances of the separate letters, especially their chronology, cannot be established with any certainty – nor is there even complete agreement about how many fragments there are – a couple of assumptions that have general agreement make this work easier to read. First, let's call it three fragments: chapters 1-7, chapters 8-9, and chapters 10-13. Second, let's say that the Corinthians received the letter containing chapters 10-13 first, reacted badly to it but then became reconciled to Paul's position, and then after hearing that report, Paul sent the letter that contained chapters 1-7 – while chapters 8 and 9 belong to a completely separate appeal. With this possibility in mind, the harshly critical tone of 10-13 makes sense as a response to what Paul is hearing other missionaries ("super-apostles," in his sarcastic designation) say about him and about his Christian message; he repudiates their criticisms and asserts that they are false apostles. When he receives a report that the congregation has struggled over this controversy but returned to Paul's perspective, he then writes chapters 1-7 to express his joy at their "repentance" and his concern that they understand why he needed to reprove them. At some other point he sends them a letter that amounts to a capital campaign solicitation (chapters 8-9), reminding them that they had promised a collection of funds to add to what he has already received to support the Christian community in Jerusalem.
What makes this letter wonderful to me is that in all of this, what stands out – if we do not get too caught up in his defensiveness – is the roller-coaster of Paul's personal feelings. He is hurt, he is insulted, he is outraged, he is contrite, he is affectionate, he is joyful, he is gracious, and throughout he is both proud of and humbled by the role of missionary of the faith to which Christ has called him. Second Corinthians offers, then, a rather intimate portrait of a man of faith feeling deeply the joys and difficulties of both being a man of faith and being part of a faith community.