2 Thessalonians 2:14 (ESV) …. “To this he called you through our gospel, so that you may obtain the glory of our Lord Jesus Christ.”
Knowing that troubled times lay ahead, Paul urged the believers in Thessalonica to ‘stand fast’ (2:15). Even as he did so, there was no doubt in his mind about the outcome. Paul wrote with serene confidence. His friends were made of the right stuff. They would pass the test. The reason for this solid confidence is revealed in his admission that he thanked God for his brothers and sisters (2:13–14).
Indeed it is worth observing that he began his declaration of thanksgiving with the word ‘But …’ He had just concluded a description of those who would not receive the love of the truth and who were on their way to hell because of it. What a relief to focus instead upon his brothers and sisters! Concentrate upon the destiny of those who reject God and we tremble.
On the other hand, it is a joy to reflect on what will become of those who love Jesus. Why was Paul confident that everything would turn out well for the members of the church in Thessalonica? The heart of it all is the principle that when God sets out to save a person, a happy ending is never in doubt. In order to make this clear, the apostle provides us with an overview of the process of salvation which stretches from eternity in the past and reaches towards eternity in the future. 
 Arthur, J. P. (1996). Patience of Hope: 1 and 2 Thessalonians Simply Explained (p. 129). Darlington, England: Evangelical Press.