But we ought always to give thanks to God for you, brothers beloved by the Lord, because God chose you as the firstfruits to be saved, through sanctification by the Spirit and belief in the truth. To this he called you through our gospel, so that you may obtain the glory of our Lord Jesus Christ.--2 Thessalonians 2:13-14 (ESV)
The third prayer I find in 2 Thessalonians is another prayer of thanksgiving. Paul thanks God for his divine election of the Thessalonian Christians. What is "divine election"? Divine election is a Calvinist or "Reformed" doctrinal persuasion that God chooses those who come to salvation; men do not choose God. Many terms are used to describe this choice; calling, election, predestination, and drawing are some of the more common terms.
I believe that the Biblical basis for divine election is sure. In addition to "God chose us as the firstfruits to be saved" in 2 Thessalonians 2:13, Ephesians 1:4 says, "he chose us in him before the foundation of the world, that we should be holy and blameless before him" (Emphasis added in all verses). In John 6:44, Jesus said, "No one can come to me unless the Father who sent me draws him". Romans 8:30 states, "and those whom he predestined he also called, and those whom he called he also justified, and those whom he justified he also glorified". It is important to realize that the doctrine of divine election does not deny the free will of man--rather, it states that without God's calling, no one would believe on Jesus Christ. Charles Spurgeon wrote, "A man is not saved against his will, but he is made willing by the operation of the Holy Ghost. A mighty grace which he does not wish to resist enters into the man, disarms him, makes a new creature of him, and he is saved."
Why do we thank God for his election? Because, without it, we would not see life. The Lord justifies only those whom he calls. Why? Because, without his regenerating work, we would not come to faith. We are at heart opposed to God. Our sinful nature, which permeates to the very deepest core of our heart, rebels against God at every point. Without a divine implantation of a new nature (the "second birth" of John 3), we would reject Jesus Christ. So, we thank God for his gracious decision to save sinners who, without His calling, would have been perfectly content to remain in their shame and defiance of God.