Paul was compelled to give thanks for the Thessalonians. The principle reason for his gratitude is that they had been chosen of God. It was a dual choosing. God chose them through the invitation of the gospel, and they were among the chosen by accepting God’s invitation.
Every Christian is called by the gospel into salvation when he obeys it. (2 Thessalonians. 2.14) The Thessalonians had obeyed the gospel and had come into this salvation, causing Paul to rejoice with thanksgiving.
They received sanctification when they became Christians and had continued to grow in it. Nothing delights a preacher more than realizing that those he has converted are becoming more and more devoted to God.
Paul praised God that the Thessalonians were set apart to live by the truth. God seeks a band of people who will honor His Word in the world.
The new converts were on their way to heaven, destined to glory. They would one day obtain and share in the glory of Jesus Christ.