Ambrosiaster

The beginning of the letter starts with the usual form of words, and the three men in whose name it is written are the same as in the first letter.

Paul adds in a fitting manner, so that they testify that great thanks must be given to God for such an infinite gift.

do not know God
This refers to the pagans who do not know that God is the Father of Jesus Christ.

in that day
The Gospel will bear witness to them in the Day of the Lord.

Deceivers have a way of counterfeiting letters under the name of some well-known person in order to mislead, so that the authority of the name may recommend something which would never be accepted on its own.

He says this so that they will not be upset by their own lack of caution and be led astray into the worship of the Devil.

so much so that he sits in the temple of God
Thus it appears that he will either be of the circumcision or at least circumcised himself, so that the Jews will have the confidence to believe in him.

presenting himself as if he were God
He will even sit in Christ's seat in the house of God and declare that he is not the Son of God, but God himself.

The mystery of iniquity began with Nero, who in his zeal for idolatry killed the apostles at the instigation of his father the Devil, and went on to Diocletian, and most recently Julian. And that, AQUINAS all the wicked who have gone before are signs of the Antichrist.

Paul says that the deception of the iniquity of his miracles is of advantage to those who will perish, because they have rejected the love of the truth by which they could have been saved through the preaching of the Apostles, they have been handed over to the Devil. They did not want to be saved and so they have been abandoned by God.

In this passage the foreknowledge of God comes through clearly. He knows the minds of all before they are born, nor is it a secret to Him of who will believe. He has known from the beginning who would become believers and which of them will increase in faith and not decrease.

In order for God's foreknoledge to remain favorable to their salvation, Paul warns them to stand and to presevere in the traditon of the Gospel. They must take care not to grow weary through idleness or sloth and so fail to complete the work of God which they have begun.

Because they had deserved it, Paul was confident that they would be kept safe by the protection of God, and so he did not doubt that they would do the things which he preached for them in the name of the Lord.

Since every congregation not everyone is obedient to the word of doctrine and some prefer their own pleasure and determination, not that of the law, Paul directs the Thessalonians to turn away from them so that they will realize their error. *A brother who is caught doing such things the bishop can bar him not only from the Sacraments but also from common intercourse with his fellows, so that when he is avoided by them he may feel ashamed and repent.

There would have been nothing wrong if he had accepted food from them for his health, because he gave them spiritual food for their souls, which also obtained for them an immortal body with glory. But there is complete freedom when someone is content with his own food and does not depend on anyone else for nourishment or money.

Then, too, while we were with you, we insisted on this to you
Paul not only taught with words but encouraged others by his deeds. It is the sign of a perfect teacher when he backs up his teaching with deeds. Those who learn know what they are hearing is true if they see that these things are not neglected by their teacher. Even if they know that what they are being taught is obviously true, if they see that it is ignored by their teacher, it will make little impression on his hearers, because actions speak louder than words.

that if anyone was not willing to work, neither should he eat
Whoever dines often at someone else's house is given over to idleness and must flatter the person who is feeding him, but our religion calls people to freedom. So Paul says that if someone refuses to eat then he does not have to work, because nobody can live without food, everyone should strive to live independently and in that way please God.

In order to convince them more easily, Paul interjects the name of our Lord Jesus Christ and encourages them to obey what is useful for them.

In his usual way, Paul encourages them with good wishes as his dearest children, praying that peace will always be with them, which is the guide to eternal life. The Lord has promised to be with those who are peaceful.