This verse finishes the question begun in 1 Thessalonians 3:9. Paul's joy over the Thessalonians did not end his praying; it made him pray more. Gill says the good news "sent them to the throne of grace to pray without ceasing." Barnes reads "Night and day" as "Constantly," and "praying exceedingly" as "much more than ordinary prayer." Paul asks for two things: to see them again, and to make up what was still lacking in their faith. Their faith was real and praised, yet Paul knew it could still grow.

Paul turns his longing straight into prayer: "Now God himself and our Father, and our Lord Jesus Christ, direct our way unto you" (1 Thessalonians 3:11). He prays that the Lord will make them "increase and abound in love one toward another, and toward all men" (1 Thessalonians 3:12), "to the end he may stablish your hearts unblameable in holiness before God" (1 Thessalonians 3:13).

Paul had been forced to leave Thessalonica suddenly; "the brethren immediately sent away Paul and Silas by night unto Berea" (Acts 17:10). Barnes notes that "he was with them but a comparatively short time before he was compelled to depart to Berea," and he thinks it "reasonable to suppose" there were many things Paul still wanted to teach them. Paul had tried to return, "but Satan hindered us" (1 Thessalonians 2:18), so he sent Timothy instead (1 Thessalonians 3:1-2).

The word "perfect" is katartisai, from katartizo. Thayer defines it as "to render artios, i.e. fit, sound, complete," and it is the same verb used for James and John "mending their nets" (Matthew 4:21). "Lacking" is husterēmata, the things still missing. Gill takes the lack first as in the grace of faith, for "there was unbelief attending them," and says it may likewise be understood of teaching, which Paul went on to supply, including the coming of Christ and the resurrection of the dead.

This question shows how God grows His people: through prayer, through presence and through teaching. Paul did not take the Thessalonians' faith for granted, and he did not shame them for what was lacking. He thanked God for what was there and asked God to complete it. Matthew Henry's Concise Commentary says that "one great end of the ministry of the word is to help faith forward."

Paul began to supply that lack in the letter itself. He goes on to teach them about those "which are asleep" and the Lord's coming (1 Thessalonians 4:13-18), the very kind of teaching that mends what is lacking. Gill notes that it is God's work to increase faith, and that the ministry of the word is the means of that increase.