Purifying ourselves is a twofold action: turning away from sin, and turning toward God. "Perfecting holiness" means that the Corinthians were to have nothing to do with paganism. They were to make a clean break with their past and give themselves to God alone.

   After this brief digression (6:14-7:1) Paul renews his appeal (cross-reference 6:13) for the Corinthians' full affection. He knows of nothing in his past conduct or instruction that can cause them to doubt his sincerity or lose confidence in him. Paul had been accused of bringing about the moral and financial ruin of innocent victims at Corinth by callously exploiting them (verse 2), and apparently some at Corinth were inclined to believe these charges. As before (cross-reference 1st Corinthians 4:4; 2nd Corinthians 4:2; 5:11; 6:3), Paul can do no more in reply than appeal to his clear conscience and the Corinthians' knowledge of his conduct and insist that the charges are groundless. (NIV Commentary - Zondervan)
   But to mention the charges did not imply that the Corinthians really believed them, nor was Paul trying to blame them for anything (verse 3a). He reminds them (cross-reference 6:11) that they occupy a permanent and secure place in his love and concern. The leveling of charges, the arrival of death, the trials of life - none of these could divorce them from his affection (verse 3b). (NIV Commentary - Zondervan)
   The situation at Corinth was not perfect and probably never would be. But Paul had grounds for great confidence and pride in his converts. In spite of all his frustrations and in the midst of all his affliction, he was filled with comfort and overflowing with joy (verse 4; cross-reference 6:10). The reason? The safe arrival of Titus in Macedonia with encouraging news about Corinth (verses 5-7). (NIV Commentary - Zondervan)

   Here Paul resumed the story that he left in 2:13, where he said he went to Macedonia to look for Titus. Though Paul still had many problems and hardships to face, he still found comfort and joy in the progress of the ministry.
   Although he expected to meet Titus when he (Paul) arrived in Macedonia, his hopes were frustrated just as they had been at Troas (2:12-13). His body (Gk. sarx, "flesh") had no rest. In 2:13 he had said that his "mind" (Gk. pneuma, "spirit") had experienced no rest at Troas. If a distinction is to be drawn between the pneuma of 2:13 and the sarx of 7:5, terms often contrasted in Paul's writing (e.g., Gal 5:16-24), the former denotes Paul in his spiritual sensitivity; the latter, Paul in his physical suffering. But more likely the terms as used here are virtually synonymous. (NIV Commentary - Zondervan)
   "Fears within" alludes to Paul's persistent apprehension about Titus's reception at Corinth, his safety in travel, and the Corinthian response to the "severe letter." "Conflicts on the outside" may point to violent quarrelling that focused on Paul or to persecution that may have happened to him in Macedonia. (NIV Commentary - Zondervan)
   It probably seemed to Paul that from the human point of view his whole future as apostle to the Gentiles was related to the Corinthians' reaction to his assertion of authority in the letter delivered by Titus. And now the nonarrival of Titus tended to confirm his worst fears). (NIV Commentary - Zondervan)
   God used three means to dispense comfort to the "downcast" apostle: the actual arrival of Titus, Titus's positive experience at Corinth, and the reassuring news he brought concerning the Corinthians' attitude toward Paul - their great "affection" for him, their "deep sorrow" over their disloyal behavior, and their "ardent concern" to defend Paul's cause and to follow his directions in disciplining the guilty party. (NIV Commentary - Zondervan)

   "My letter" refers to the third letter (now lost) that Paul had written to the Corinthians. Apparently it had caused the people to begin to change.
   "Godly sorrow brings repentance that leads to salvation" refers to the sorrow for our sins that results in changed behavior. Many people are sorry only for the effects of their sins or for being caught ("worldly sorrow"). Compare Peter's remorse and repentance with Judas's bitterness and act of suicide. Both disowned Christ. One repented and was restored to faith and service; the other took his own life.

   It is difficult to be confronted with our sin, and even more difficult to get rid of sin. Paul praised the Corinthians for clearing up an especially troublesome situation. Do you tend to be defensive when confronted? Don't let pride keep you from admitting your sins. Accept correction as a tool for your growth, and do all you can to correct problems that are pointed out to you.

   Paul's principal aim in writing the "severe letter" was for the Corinthians to recognize "before God" how devoted to their spiritual father they really were (cross-reference 2:9) and to ensure their future loyalty to Paul. This statement of his aim was likely influenced by his knowledge of the letter's outcome. At the time he actually wrote that letter he was unsure of Corinthian loyalty and hence was restless (2:12-13; 7:5). But since God had prevented the letter from making the Corinthians resentful (verse 9b), Paul was encouraged (verse 13a). (NIV Commentary - Zondervan)
   Paul also hints at two subsidiary aims in this letter: the punishment of the guilty party (cross-reference 2:6, 9) and the vindication of "the injured party." The offender was probably not the man guilty of incest, but an anti-Pauline intruder in the church; the injured party was likely Paul himself. (NIV Commentary - Zondervan)

   Through the "godly sorrow" of the Corinthians, Titus was as relieved and encouraged as Paul. He apparently had little occasion before his visit to Corinth as bearer of the "severe letter" to form an independent judgment about the Corinthians; so he was dependent on Paul's glowing recommendation. This would suggest that this visit was his first one (though he may have had a brief earlier visit to organize the collection; see 1st Corinthians 16:1-2). Titus seems to have ventured on this visit with some trepidation. But now that the visit was over, "his spirit has been refreshed." (NIV Commentary - Zondervan)
   Paul's relief stemmed from the fact that his generous assurances to Titus about the Corinthians had not proved unfounded and therefore embarrassing to him (verse 11). On the contrary, just as his own truthfulness had been vindicated at Corinth (cross-reference 1:18-20), so also his boasting about them had now proved fully justified. (NIV Commentary - Zondervan)

   The Corinthian Christians had originally received Titus "with fear and trembling," afraid (cross-reference verse 11) that they would fail to meet their obligations toward an envoy from Paul. But they had all readily complied with demands Titus had made of them. As Titus recalled their obedience and respectful deference to him, his affection grew all the warmer (verse 15). This gave Paul good reason for complete confidence in the Corinthians (verse 16) and a secure base from which to propose the completion of the collection (chapters 8-9). (NIV Commentary - Zondervan)