Paul exhorts the Thessalonians to walk with and please God by following the will of God, which is that they live set apart, holy, sanctified. To be sanctified is to walk in the ways ordained by God. This sanctification looks like living a sexually pure life, using self-control and not behaving like the pagan Greeks who followed their lusts. Sexual immorality is exploitative—it hurts other people or takes advantage of them. If we reject sexual purity, we are rejecting the ways of God and His Spirit. Sexual purity is fundamental to living a sanctified life.
Sanctification also looks like minding our own business, working hard at our jobs and serving rather than being a burden on others. Paul commends the Thessalonians on how well they show love for one another, and urges them to increase in their love even more. Living in love is something that requires constant investment, there is no coasting. Living as servants to one another rather than exploiting one another exhibits this bounty of love, which he encourages.
Finally, he comforts the Thessalonians about the return of Jesus. They were under the impression that if any of them died before Jesus’ second coming, they would remain dead and would miss out on spending eternity with Jesus. Paul clarifies that dead believers will be resurrected and meet Jesus in the air at His coming, and living believers will subsequently join them, and all will be with the Lord forever.