1 Thessalonians 5

Sin
We should surely understand those "in darkness" to be associated with sin. I assume Paul largely has in mind those who are not believers, so there is an association between sin and experiencing God's wrath here (5:9).

So believers are thus "sons of light" (5:5), Qumran imagery. That means they are "awake" and looking for Christ's return (5:4), when instead of wrath they will get "salvation" (5:9). Believers put on the breastplate of faith and love and the hope of salvation as a helmet (5:8). Faith and love and hope are dispositions that imply actions. Thus encourages them to move in a certain direction attitudinally and behaviorally.

Paul gives other exhortations at the end of the letter. He wants them to be at peace. He insists that the lazy must work. Again, he makes ethical demands. If he believed in eternal security, the idea shows up no where here. From this letter, we get every implication that believers are to live a certain way and that not living that way can have serious consequences.

In 5:23, Paul expresses his desire that the God of peace "sanctify" them completely. What that entails is seen by the state in which he then desires them to remain thereafter--that their entire spirit, soul, and body be preserved blameless. To sanctify them thus implies that they become blameless, something Paul says God, the one who calls them, will do because He is faithful (5:24). Peace surely also has something to do with God's sanctification of the Thessalonians, for otherwise why would Paul mention it here?

The picture that emerges from 1 Thessalonians with regard to sanctification is that to be holy as a church and as individuals in that church is to be operating in a blameless way in relation to God. For individuals, it implies things like abstaining from sexual immorality and not being idle. For the collective church it implies loving one another and being at peace with each other.

As far as Paul's view of sin in 1 Thessalonians, it seems to be actions that are wrong. Paul assumes what that might be. It is what he understands to be wrong. That includes worshipping the wrong god or opposing the Christ or being sexually immoral, etc. Although he only uses the word sin once in the letter, and that of the Jews who oppose Christ, we'll work with this operating sense of sin until proven otherwise.