the son of perdition is the body of people who depart from God to return to their sins and lusts of their flesh- going back from salvation, into perdition.
Then because they did not have a love for truth but had pleasure in unrightousness they become inhabited by the spirit of apollyon from the pit - this is the mystery of iniquity.
Just as Christ is to be manifest in us to the world shining His light and conforming us into His holy Image, so too this wicked spirit will enter all these who fall away and bring darkness to the earth and conform them into the image of wickedness.
God will send them strong delusion because they did not love the truth but loved the lusts of their flesh.
These will depart from us and fall away. Remember that WE are the temple of God. These people sit amoung us hidden for now, they are growing up with us but it is not yet seen what they really are. That they feed the lusts of their flesh in private. Just as judas was a thief of the brethern and stole from the common purse- just like esau sold his birthright for meat, just like cain hated his brother because his sacrifice was of the world, just like balaam loved the wages of unrightousness and betrayed Gods people because of it- too these too sit in the temple of God- the body of Christ, and defile their temples. Those who do this God says he will destroy. They sit at love feasts with us but inwardly are ravenous wolves.
In the end times they will fall away and depart from us which will reveal- or make manifest- that they are not of us, that they are antichrist and they will be conformed into the image of their father the devil while the Lord is conforming us into the image of His Son Jesus Christ!