Ambrosiaster

We give thanks to God always for all of you:
Since all things come from God, they first offer thanks for their progress, because God has been pleased not only to give His gift to people who were still in bondage to sin, but also to adopt them as His children, at the same time helping them to put up with all suffering so that they might become worthier.
Keeping the memory of you:
This is because they have grown in the work of faith more than other churches and by God's grace were perfecting the work which they had eagerly begun.

In order to indicate that he showed all humility among the Thessalonians and in order to adopt a gentle approach in dealing with them, Paul makes it clear that none of the powerful means by which they have been brought to faith is lacking. His preaching was accompanied by signs and wonders and was empowered by the Holy Spirit. It embodied the fullness of truth rather than fantasy. This could be seen in the teaching of grace, in the quality of discourse among them and in the healing of the sick. By these they could be seen to be the true heirs of the gift of God in the promises to Abraham.

Those who, eager to believe, suffer insults and injuries from their fellows, are precisely those who may be called imitators of the apostles and of the Lord himself. He suffered the same things from the Jews, as did the apostles who endured persecution as they pursued their faith in God.