Albert Barnes Commentary 2 Thessalonians 2:13

"But we are bound to give thanks to God always for you, brethren beloved of the Lord, for that God chose you from the beginning unto salvation in sanctification of the Spirit and belief of the truth:" — 2 Thessalonians 2:13 (ASV)

But we are bound to give thanks always to God for you. (See Barnes on 2 Thessalonians 1:3.)

Because God has from the beginning chosen you to salvation. The following important things are affirmed or implied here:

1. That God had chosen or elected them (eileto) to salvation. The doctrine of election, therefore, is true.
2. That this was from the beginning (ap’ archēs), that is, from eternity (John 1:1; Ephesians 1:4; Ephesians 3:9–11). The doctrine of eternal election is, therefore, true.
3. That this was the choice of the persons to whom Paul referred. The doctrine of personal election is, therefore, true.
4. That this is a reason for thanksgiving. Why should it not be? Can there be any higher ground of praise or gratitude than that God has chosen us to be eternally holy and happy, and that he has from eternity designed that we should be so? Whatever, therefore, may be the feelings with which those who are not chosen to salvation regard this doctrine, it is clear that those who have evidence that they are chosen should make it a subject of grateful praise. They can have no more exalted source of gratitude than that they are chosen to eternal life.

Through sanctification of the Spirit. Being made holy by the Divine Spirit. It is not without respect to character, but it is a choice to holiness and then to salvation. No one can have evidence that he is chosen to salvation except as he has evidence that he is sanctified by the Spirit. (See Barnes on Ephesians 1:4.)

And belief of the truth. In connection with believing the truth. No one who is not a believer in the truth can have evidence that God has chosen him.