In the last part of 1 Thessalonians 4, Paul focused on offering comfort to those who grieved over deceased fellow believers. He was selective in what he presented regarding the return of Christ because he had a narrow objective of putting to rest any concern that those who have died in Christ will somehow be at a disadvantage compared to those who remain when Christ returns.
In the first part of 1 Thessalonians 5, Paul then expands on his treatment of the return of Christ, also called the Day of the Lord. It seems that his primary objective in these verses is to bring the attention of his readers from the hope of the future resurrection to the present necessity of living in light of their faith and hope. To do this, he reminds them of who they are: sons of light, and then, because of who they are, he exhorts them to live accordingly. Paul often uses this idea that you do this or that because of who you are in Christ, because it properly grounds our actions in the grace of God.
As Paul reminds us who we are in Christ, he offers a strong word of assurance: “For God has not destined us for wrath, but to obtain salvation through our Lord Jesus Christ, 10 who died for us so that whether we are awake or asleep we might live with him” (1 Thess 5:9-10 ESV). But for anyone who struggles with assurance of salvation, this is an audacious statement. After all, the Day of the Lord is the day when Christ comes again to judge the living and the dead. Moreover, Scripture elsewhere states that everyone will be judged according to their works. To give just one example, Paul writes, “For we must all appear before the judgment seat of Christ, so that each one may receive what is due for what he has done in the body, whether good or evil” (2 Cor 5:10 ESV).
How, then, do we make sense of this strong word of assurance in light of the whole testimony of Scripture? More importantly, how can this word of assurance be appropriated by those who struggle with assurance? Herman Bavinck raises and addresses this issue with his usual helpfulness:
Scripture does indeed say that all humans without distinction, hence also believers, must appear before the judgment seat of Christ. But it also attests that those who believe in him are not condemned and do not come into judgment, for they already have eternal life (John 3:18; 5:24); that the believers who have died are already with Christ in heaven and clothed in long white garments (2 Cor 5:8; Phil 1:23; Rev 6:11; 7:9,14); and that Christ is coming to be glorified in his saints and to be marveled at among all who believe (2 Thess 1:10). Before pronouncing his verdict on the evil angels, on the anti-Christian world, and on barbaric peoples, Christ has already positioned the sheep at his right hand and is surrounded by his angels and his saints. This is also evident from 1 Cor 6:2-3, where Paul expressly states that the saints will judge the world and the angels. This statement may not be watered down into an active endorsement by believers of the judgment Christ pronounces over the world and the angels, but as the context shows, specifically indicates at the saints will participate in Christ's judgment of the world and the angels.1
The power of the idea expressed here by Bavinck is that believers do not face a final exam, as it were, but are judged as righteous on account of the righteousness of Christ imputed to us by faith from the moment we repent and believe. It is not as though our conversion is the mid-term exam, covering our lives up to that point, and the final judgment covers everything else from the mid-term through the end of the course of our lives. No, while everyone does appear before the judgment seat of Christ on the Day of the Lord, believers do not come into judgment because God has already judged all of our sins, past, present, and future, in Christ and acquitted us.
Therefore, the Day of the Lord is not something to be feared by believers who might wonder if they’re going to make it past the final exam, but something to long for since the fullness of our salvation will be realized on the Day of the Lord.