The second letter to the Thessalonians is used to support the Calvinistic understanding of election: But we are bound to give thanks alway to God for you, brethren beloved of the Lord, because God hath from the beginning chosen you to salvation through sanctification of the Spirit and belief of the truth (2 Thessalonians 2:13). The meaning of the Greek phrase “from the beginning” is in dispute with some Greek scholars. Jamieson, Fausset and Brown state that:

Some of the oldest manuscripts read as English Version, but other oldest manuscripts and Vulgate read, "as first-fruits." The Thessalonians were among the first converts in Europe (compare Romans 16:5, 1 Corinthians 16:15).¹

If it were granted that the phrase “from the beginning” was the correct understanding, it deserves consideration. It cannot mean initial salvation upon believing, for then it would say that “God from the beginning chose you to belief of the truth through sanctification and belief of the truth”. This would not make sense. Their belief is the means by which they are saved. In verses 10-12 Paul says that God appoints a strong delusion upon those who reject the truth, and in parallel he appoints salvation to those who accept the truth. What is the salvation to in this passage? Verse 14 provides the answer: to the obtaining of the glory of the glory of our Lord Jesus Christ. That is, final salvation, or glorification is in view. Paul was reminding the Thessalonian believers about the future they have to look forward to because of their faith.