Finally, both the nature of the argument (which brings in the presence of the Spirit in their lives as the clincher) and the emphasis on the “presentness” and “indwelling” nature of the Spirit as gift point to the Spirit as God’s effectual empowerment in the struggle against sin. This does not mean that the Spirit guarantees perfection — hardly so — but it does mean that one is left without argument for helplessness. As Plummer (correctly) asserts: “This [gifting of Spirit] transformed their whole life; and it put an end to the pagan plea that [people have] no power to resist impure desires” (63). For Paul the presence of the Spirit was not simply God’s gift as an option against sin; nor would he have understood the Spirit as present but ineffectively so. To the contrary, the dynamic that makes Paul’s argument against sexual impurity possible is the experienced reality of the Spirit. Thus for Paul the Spirit is not only the key to becoming believers (1:6), but is the power for truly Christian behavior, and therefore makes disobedience on their part a difficult thing to argue for.

—Fee, Gordon, The First and Second Letters to the Thessalonians, NICNT, p. 155. This is a phenomenal commentary. I love Gordon Fee. This paragraph is in conclusion to 1 Thessalonians 4:8, where Paul warns the Thessalonians about rejecting his teaching. If they reject his teaching, they reject not man, but God who gave them his Spirit.