V 1.
And now, brothers, about the coming of our Lord Jesus Christ and his gathering of us to himself:
NLT.  And now, brothers and sisters, let us tell you about the coming again of our Lord Jesus Christ and how we will be gathered together to meet him.

Paul wrote this letter to clear up some ideas about the return of Jesus that were going around in the church at Thessalonia and getting people anxious and worked up.  He calls them brothers, because he isn't coming down hard on them, but is going to gently explain as to equals, which he knew they were.  Paul did not put himself above other Christians.  Christian leaders need to be wary of appearing to feel they are superior in some way to those they lead.  We are all equal in the sight of God and each of us has our own gifts from him to be used in serving his church.  Paul used his in the best way he could.

Quest Study Bible note, p 1634. The coming of the Lord refers to the day Jesus returns to the earth to gather believers and judge unbelievers.  It may also include some of the dramatic events that preceded the actual coming of the Lord: plagues, famines earthquakes. (mentioned by Jesus in Matthew 24: 7 - 8, 30 - 31.)

V 2.
I beg you, do not suddenly lose your heads or alarm yourselves, whether at some oracular utterance, or pronouncement, or some letter purporting to come from us, alleging that the Day of the Lord is already here.
NLT.  Please don't be so easily shaken and troubled by those who say that the day of the Lord has already begun.  Even if they claim to have had a vision, a revelation, or a letter supposedly from us, don't believe them.

From time to time we hear even today, reports of people predicting the end of the world. Although, in Matthew 24 and in Mark 13, we have reports of Jesus describing what will happen before he returns in glory, he says (Matthew 24: 36) that only God knows when this will happen.  Jesus described some events that have happened over and over again in the history of the earth - and will probably happen again before Jesus returns - earthquakes, plagues, famines, wars, etc. Jesus said that it is our job not to worry about all these possibilities, but to remain firm in our faith, to trust God and be faithful to him so that we will be ready whenever the great day comes.  Paul is re-inforcing here what Jesus said.  Don't worry about it all; don't listen to people who try to alarm you

New Bible Commentary, p 1288.  Now some people were saying that the day of the Lord has already come (NIV).  For tehm the 'day' was an extended period which would culminate in the coming of the Lord, and probably they saw their present persecution as the last stages of opposition to the kingdom of God.  Their teaching was unsettling to people, causing excitement as well as uncertainty, and no doubt distracting people from normal living.  In support of their claim they said that Paul himself had taught it.......

Drane, J., Introducing the New Testament , p 305.  Fanatics of some kind had taken advantage of Paul's mention of the parousia (expected return) of Jesus in his earlier letter, and used the occasion to put their own point of view on the subject.... The precise meaning of the suggestion that 'the day of the Lord has come' is difficult to establish.  In 1 Corinthians we find mention of people who thought that the resurrection (which was associated with the end of things, and the parousia of Christ) had already taken place.  On the basis of this belief they indulged in all, kinds of immoral practices. (1 Corinthians 15: 12-58)

V.3, 4.
Let no-one deceive you in any way whatever.  That day cannot come before the final rebellion against God, when wickedness will be revealed in human form, the man doomed to perdition. He is the Enemy.  He rises in his pride against every god, so called, every object of men's worship, and even takes his seat in the temple of God claiming to be a god himself.
TEV.  Do not let anyone fool you in any way.  For the Day will not come until the final Rebellion takes place and the Wicked One appears, who is destined to hell.  He will oppose everything which men worship and eveedryhting which men consider divine.  He will put himself above them all, and even go in and sit down in God's temple and claim to be God!

Like the warnings made from time to time about the end of the world coming on a certain date, we've aslo had certain evil people in history called the 'Anti-Christ', causing some to wonder whether this might be the person Paul talks about here, the man of lawlessness (NIV, NLT).  We must be careful, understanding that it is not our job to make this judgement, but to trust that God will bring things to a head in his own good tiime and that he will be triumphant whenever it happens.  Satan must have a field day, when he sees people panicking about predictions of the end of the world!  It is our job, as Christians, to spread the true Gospel and to pray that people will learn to trust in God and turn to him in times of uncertainty.

Drane, J., Introducing the New Testament , p 305.  Paul goes on to emphasise here that in his view the parousia of Jesus and all that it entails was not an event that could take place invisibly or mystically (which would need to be the case if it had already happened).  On the contrary, he fixes his own hope firmly in history by making it clear that certain historical events connected with 'the man of lawlessness' who would herald the return of Christ.

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V 13, 14
But we are bound to thank God always for you, brothers beloved by the Lord, because from the beginning of time God chose you to find salvation in the Spirit that consecrates you, and in the truth that you believe. It was for this that he called you through the gospel we brought, so that you might possess for your own the splendour of our Lord Jesus Christ.

In letting the people know that he gives thanks for them and for the strength of their beliefs, Paul was reminding the Thessalonians that they are loved by God and that it has been his plan from the very beginning of creation that they should be saved by their faith and know him through the work and power of the Holy Spirit.  It is so important that we remember these things at all times, so we remember to focus our life, our trust and our hope in God, and not on ouselves.

V 15.
Stand firm then, brothers, and hold fast to the traditions which you have learned from us by word or by letter.

Paul asks the Thessalonians to remain true to the things he has taught them and not be swayed by other people whose credntials they don't know.  They are to trust what Paul has taught them either in person or by letters from him that they have seen and know for sure were written by him.  They would know this, I assume, because of who delivered the letter, as well as the style and the fact that what it taught would be consistent with what he taught them when he was there

We need to be sure that the people we listen to are genuinely teaching us the word of God.  We can only know that by checking out what they say against what the Bible tells us.  This means that every Christian needs to know and understand what the Bible teaches if we are not to be led astray by false ideas.  It's up to us to do something about that - not wait for a priest, pastor, minister or some other 'expert' to tell us what the Bible says.

V 16, 17
And may our Lord Jesus Christ himself and God our Father, who has shown us such love, and in his grace has given us such unfailing encouragement and such bright hopes, still encourage and fortify you in every good deed and word!
The Message.  May Jesus himself and God our Father, who reached out in love and surprised you with gifts of unending help and confidence, put a fresh heart in you, invigorate your work, enliven your speech.

New Bible Commentary, p 1289.  Paul backs up his exhortation with a prayer, expressed in the third person form.  It conveys incidentally that Jesus (named first) and the Father are the joint source of spiritual blessings.  It reminds the readers that God loves them and so has given them encouragement and hope for the future - despite the fearful events mentioned earlier in the chapeter.  It asks that God will continue to encourage them in their hearts and make them resolute in acting and speaking in ways which demonstrate the reality of their faith.

What a wonderful prayer!  And a great pattern for us to follow in praying for our fellow Christians.