Paul thanks God for their continuing faithfulness: “How can we thank God enough for you in return for all the joy we have in the presence of our God because of you?” Paul has finished recounting his history with the readers, but the relationship is not finished. It continues by means of this letter, but Paul also hopes that it continues with personal contact.
Just as Paul turned his joy toward God in thanks, he also turns his hopes for the future toward God in prayer: “Night and day we pray most earnestly that we may see you again and supply what is lacking in your faith.”
What was lacking in their faith? Perhaps Paul means that he wants to tell them more about the content of their faith — faith in the sense of “the Christian faith.” Judging by this letter, they lack very little; Paul does not criticize what they are doing.
He prays that he will be able to visit them: “Now may our God and Father himself and our Lord Jesus clear the way for us to come to you.”
And he prays for their spiritual growth: “May the Lord make your love increase and overflow for each other and for everyone else, just as ours does for you. May he strengthen your hearts so that you will be blameless and holy in the presence of our God and Father when our Lord Jesus comes with all his holy ones.”
Does “blameless” mean that Paul expects them to be morally perfect by the time Christ returns? Paul’s prayer here means about the same thing as “I hope that you achieve everything that God wants you to achieve.” It is a sentiment, not a prophecy, and not a formula for salvation.
First Thessalonians is a letter of encouragement, not a letter of doctrinal instruction, and we should not try to squeeze doctrine out of passages in which Paul is not trying to explain a doctrine. Some parts of the Bible are doctrinal, but other parts are more like a story, and some are motivational. God inspired every type, and we need to receive it the way it is, not try to force it into something else.