Good Scripture!
If you read the context of 2Thess.2, from vs.3 on, you’ll see vss.11&12 applies to the “lawless”, those acting ungodly.
Christendom has been “lawless” since its inception: for instance, killing their brothers. So it’s no stretch to expect that much of what they “believe” are lies, and teach them.
But not all of those claiming to follow Jesus, are involved in “lawless” deeds. Some are sincere, and keep themselves “without spot from the world.” -James 1:27.
No it refers to all who don't believe the truth...
"Therefore God sends upon them a strong delusion, to make them believe what is false, 12 so that all may be condemned who did not believe the truth but had pleasure in unrighteousness."
This is significant for two reasons. Firstly because according to Paul, God chooses the believers he wants, before the world began (Rom.8:29-30, Eph.1:4-5, 2 Tim.1:9) and also He chooses whom he will condemn by sending them delusion, not to believe. In other words, it is irrelevant if you believe in Jesus or not. If you were not chosen by God (before the world began) you are bound to be condemned.
Secondly, it means that all Buddhists, Muslims, Hindus, Jewish, and all other religions, have received delusion from God, not to believe.
That's Paul's deterministic view, in full contrast with Jesus and Apostles teachings, no matter how hard can one try to smooth it out.