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Many (I think the majority) argue that both the neuter expression τὸ κατέχον here in verse 6 and the masculine ὁ κατέχων in verse 7 refer to the "Holy Spirit" (τὸ πνεῦμα τὸ ἅγιον). This is grammatically very problematic, not to mention that the "Holy Spirit" is nowhere mentioned in the immediate context.
In fact τὸ κατέχον refers to τὸ μυστήριον τῆς ἀνομίας in verse 7. So in a very real sense many readers put evil for good in their understanding of verse 6, confusing the spirit of lawlessness ,which issues from Satan, with the Holy Spirit of God.
The Westcott and Hort punctuation and apparatus follows, which I've recreated here (hopefully without typos; italics are theirs) for their interpretation of that ὅτι ἐὰν... Notice they put a "-" at the end of it, explaining the missing apodosis as the statement breaking off just before Paul asks them if they remember him telling them something about this in person. Interesting, especially in light of the accusation of false letters floating around, which is itself especially interesting in light of this being a bit of a pull-back from 1Thes.
I personally understood the τὸ κατέχον as "the blockage" or "the holdup" and "ὁ κατέχων" as "the one holding back", but it seems open for debate. If I were going to try to make your case, I would start with the "ὁ ἀντικείμενος" coming just before. Is that exactly opposite or exactly the same as ὁ κατέχων?
According to my understanding ὁ ἀντικείμενος is the anti-Christ (i.e. the God-man). This is the one who sits in the temple of God and proclaims himself to be God (i.e. ὁ ἄνθρωπος τῆς ἀνομίας, ὁ υἱὸς τῆς ἀπωλείας). τὸ κατέχον is the mystery of lawlessness (τὸ μυστήριον τῆς ἀνομίας), which is the power of Satan. ὁ κατέχων is Satan, that ancient Dragon . A time will come when Satan no longer will have access to Heaven before God's throne , that is, when he is "gone out of the midst" -- essentially when he is confined to the earthly sphere (see Rev. 12:7-12). It is then that the man of lawlessness (the second person of the Trinity, as it were) will also be revealed upon the earth, culminating with the arrival of Jesus and the ultimate destruction of the anti-Christ by him (see 2 Thes. 2:7-8).
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