2 Thessalonians 2:10 and all the ways that wickedness deceives those who are perishing. They perish because they refused to love the truth and so be saved. 11 For this reason God sends them a powerful delusion so that they will believe the lie 12 and so that all will be condemned who have not believed the truth but have delighted in wickedness.
If you look at the way it's being written, it is written in the present, on-going tense, not future. That means it's going on now in the present, not just later after the mark of the beast is instituted.
Paul believed it could happen in his time when he wrote those words, so he's talking about what happens to people in general who refuse to love the truth - they end up believing the lie. People have been rejecting God and His truth since the beginning and get dealt by God in this way. You don't need the mark of the beast for that. Just ask the Pharisees, and all the other religious leaders that thought more of their traditions and false teaching (which they don't see as false teaching - see how that strong delusion works?).
2Th 2:3 Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition;
2Th 2:4 Who opposeth and exalteth himself above all that is called God, or that is worshipped; so that he as God sitteth in the temple of God, shewing himself that he is God.
This is speaking of the antichrist which is in the future.
2Th 2:7 For the mystery of iniquity doth already work: only he who now letteth will let, until he be taken out of the way.
2Th 2:8 And then shall that Wicked be revealed, whom the Lord shall consume with the spirit of his mouth, and shall destroy with the brightness of his coming:
2Th 2:9 Even him, whose coming is after the working of Satan with all power and signs and lying wonders.
Which the antichrist will be revealed to the world and Satan will deceive the world who does not love the truth which is in the future.
2Th 2:10 And with all deceivableness of unrighteousness in them that perish; because they received not the love of the truth, that they might be saved.
2Th 2:11 And for this cause God shall send them strong delusion, that they should believe a lie:
2Th 2:12 That they all might be damned who believed not the truth, but had pleasure in unrighteousness.
This happens after the antichrist is revealed which Satan deceives the world which happens in the future.
The man of sin is revealed and then they are given strong delusion.
The antichrist spirit is working in the world but this is talking about the antichrist/man of sin being revealed which Satan deceives the world and then the strong delusion is given which happens in the future.
Before this time there is no mention of strong delusion.
Or even plain ole' run of the mill delusion.
