For the mystery of iniquity doth already work..2Thes.2:7
And with all deceivableness of unrighteousness in them that perish; because they received not the love of the truth, that they might be saved. And for this cause God shall send them strong delusion, that they should believe a lie: That they all might be damned who believed not the truth, but had pleasure in unrighteousness. 2Thes.2:10-12
Those in the world are not “with” God. They are not professing Christ. They can’t fall away from Him or rebel against Him as His people; they never had Him or claimed Him to begin with. The apostacy happens in the Church. The mystery of iniquity doth already work in the Church. That is the mystery: it happens in the Church. God allows satan to come with all the deception of wickedness in them that perish in the visible Church because they do not receive the love of the truth that they might be saved. For this reason God sends the strong delusion to them within Christianity– in order that they might believe what is false–that they should believe a lie. That they all might be damned who believe not the truth, but have pleasure in unrighteousness. This wickedness they have pleasure in is not referring to sexual immorality. The wickedness is having pleasure in believing in lies that are an abomination to God. It is having pleasure in idol worship.
But I thank God for you, brethren beloved of the Lord, because God has chosen you from the beginning for salvation through sanctification by the Spirit and faith in the truth.