Obeying implies there is a command to obey as a prerequisite to salvation. Now we’re starting to venture a bit towards a works-based salvation which is an entirely different animal. As you pointed out in your first post, 1 Corinthians 15:2-3 briefly summarizes the gospel of Christ and so does 1 John 5:13. Salvation is based entirely on someone’s faith in Jesus Christ.
Also, I might add, faith is not considered to be a work of self-effort in order to earn salvation.
Faith is sourced from the Word of God:
Romans 10:17
17So then faith
cometh by hearing, and hearing by the word of God.
Faith is a spiritual gift of the Holy Spirit:
1 Corinthians 12:9
9To another faith by the same Spirit; to another the gifts of healing by the same Spirit;
Salvation is by grace through faith. Graces means someone has favor with God even when they don’t seem like they should deserve it. And this also says faith is not a work that we do.
Ephesians 2:8-9
8For by grace are ye saved through faith; and that not of yourselves:
it is the gift of God:
9Not of works, lest any man should boast.
So 2 Thessalonians 1:8-9, regarding those who do not obey the gospel, is about those who know what the gospel is, had a chance to decide to disobey it, and then hardened their hearts and rejected the gift of the Holy Spirit. All of this can happen as quickly as a blink of an eye without people fully realizing the grave mistake they just made.
2 Thess. 1:8-9 is talking about the rank and file unbeliever, though they may be a morally and ethically “good person”, is actually spiritually dead, will not receive eternal life, and will be destroyed in the afterlife per John 3:16.