he who opposes and exalts himself against all that is called God or that is worshiped; so that he sits as God in the temple of God, setting himself up as God.
The portion of the Christian church in Paul’s view would be the non-genuine Christians who compose Christendom. "Christendom" refers to all professing Christians, genuine and non-genuine. Such a departure had begun in Paul’s day (1 Timothy 4:1-3; 2 Timothy 4:3-4; James 5:1-8; 2 Peter 2; 2 Peter 3:3-6; Jude). However it had not yet reached the proportions predicted to characterize "the apostasy" about which Paul had instructed his readers when he was with them (cf. 2 Thessalonians 2:5). When the Rapture takes place and all true Christians leave the earth, this apostasy will overwhelm the human race.