In the opening verses of 1 Thessalonians, Paul declares his knowledge of God's divine choosing of the Christians in Thessalonica, "For we know, brothers and sisters beloved of God, that he has chosen you, because our message of the gospel came to you not in word only, but also in power and in the Holy Spirit and with full conviction" (4-5). Paul's statement of God's election of the Thessalonian Christians is clear. He does not say that he merely thinks or suspects that God has chosen the Thessalonians. No, Paul and co-authors declare their knowledge of God's choosing of the Thessalonians. They point to the power of God displayed in the preaching of the gospel which resulted in conviction of sin.
This bold statement of God's electing purposes is particularly interesting in light of what Paul says in chapter 3. Having heard that his Thessalonian converts were experiencing some sort of tribulation or persecution, Paul says that, "For this reason, when I could bear it no longer, I sent to find out about your faith; I was afraid that somehow the tempter had tempted you and that our labor had been in vain" (5). Paul here expresses his fear that the Thessalonians had succumbed to temptation and forsaken the faith which would have made his evangelistic work among them empty. If they did not stay the course, his would would have been in vain. Paul conveys a deep sense of worry and concern over the state of Thessalonians' faith.
This is so striking because it doesn't seem to fit with what Paul says about the Thessalonians in 1:5. If Paul is so confident that God has chosen the Thessalonians, why is he so fearful that they may forsake the faith? These texts demonstrate that we cannot simply conclude that God's electing of the Thessalonians ensures their final perseverance. Whatever Paul means when he speaks of God's choosing (ekloge), he cannot possibly think of it as unconditional. If Paul really thought that the Thessalonians had been chosen unconditionally, then he would have no reason to be concerned about their faithlessness. Rather, we must conclude that Paul understood God's choosing of the Thessalonians to be conditional upon their continued perseverance in faith. In Paul's mind, one can be chosen by God and still potentially fall away.