‘But we are bound to give thanks to God always for you, brothers and sisters beloved of the Lord, for that God chose you from the beginning unto salvation in sanctification of the Spirit and belief of the truth, whereunto he called you through our Gospel to the obtaining of the glory of our Lord Jesus Christ.'
Having considered the man of sin, and the destiny of those who follow him, Paul now assures the Thessalonians that he realises that they are not of that number. Rather are they beloved of the Lord (that is, the Lord Jesus Christ). And that immediately brings him ‘under an obligation' to give thanks for what has caused them to be so, God's electing grace.
‘For that God chose you from the beginning.' Compare Eph 1:4; 1 Peter 1:1-2. From the beginning means ‘before the foundation of the world' (Ephesians 1:4). From the beginning of His act of creation, even before the world came into being, He chose His own. Their salvation was not an afterthought resulting from failure, but the specific purpose of God which He would bring into fulfilment. They were ‘chosen according to the foreknowing of God' (1 Peter 1:1-2).
Some good texts have ‘as firstfruits' rather than ‘from the beginning', and if this reading were accepted it would mirror the teaching in Revelation 14 that God's people are his firsfruits, while the remainder of the world awaits judgment.
‘Unto salvation.' Man's sin, man's fall, all was beforehand known to God. And His purpose to save His own was part of that foreknowing. He chose them unto salvation. He, as it were, entered into personal relationship with them beforehand long prior to making His effectual call. This was then to be accomplished by two means, sanctification in the Spirit, which is God's side, and belief of the truth, which is the resulting response of man.
Here sanctification of the Spirit begins in the working of truth in a man's heart (as against the ‘working of error' - 2 Thessalonians 2:11), which results in belief of the truth, continues in giving new life through the new birth (John 3:6; Titus 3:5; James 1:18; 2 Peter 1:4; Galatians 4:19), and then continues on as He works within them to will and to do of His good pleasure (Philippians 2:13; 1 Corinthians 1:8; Jude 1:24), until He finally presents them perfect before Him, holy and without blemish (Ephesians 5:27; Colossians 1:22; Jude 1:24). And the result in such men is a continuing response of belief and trust in the truth of the Gospel, seen as a wholehearted response to Him..
‘Whereunto He called you through our Gospel.' This looks back to all the blessings described, salvation, sanctification in the Spirit and belief of the truth. They were effectually called through the work of the Spirit to these things. And the means of their calling was the Good News presented by Paul and his companions. In Paul ‘calling' is always seen as effectual (compare 1 Thessalonians 2:12; 1 Thessalonians 5:24; 2 Timothy 1:9).
But what was to be the end result? ‘The obtaining of the glory of our Lord Jesus Christ'. In that day we will see His glory, and we shall be like Him, for we will see Him as He is (1 John 3:2). The Christian's end is glory, ‘a far more exceeding and eternal weight of glory' (2 Corinthians 4:17; Romans 8:17-18; Romans 8:30; 1 Corinthians 15:43; Ephesians 5:27; Philippians 3:21; 1 Thessalonians 2:12). We will appear with Him in glory (Colossians 3:4). As God has glorified Jesus, the representative man, so will He glorify us. And the process has already begun (2 Corinthians 3:18; John 17:22).
Note in all this the working of the whole Godhead. ‘God chose you -- in sanctification of the Spirit -- to the obtaining of the glory of our Lord Jesus Christ'. None of this is due to our merit. It is because God chose us and worked His will within us that we will share the glory of our Lord Jesus Christ.