You have to understand "Christendom". Within the Kingdom of the Heavens there co-exists the true and the false. As we approach the end, the false part of "Christendom" will evidence a greater and greater departure from even the semblance of sound doctrine.
The distinctions between those who are truly born-again and the false-professors will become more and more apparent. The way of Cain (coming to God by a self-made means and not as proscribed by God) will gain strength and will separate and hate those of the way of Abel (those who come by the blood sacrifice of Christ alone).
This may be an event that happens clearly in the Tribulation. I say that because the verse quoted is in line with the revelation of the "man of sin". As with all Tribulation prophecy, we will not see it come to fruition, but we can see the beginnings of sorrow.
7 For the mystery of iniquity doth already work: only he who now letteth will let, until he be taken out of the way.
So, even in Paul's day, the mystery of iniquity which will culminate in the rise of Antichrist and the False Prophet had begun. In the case of Thessalonica, it was manifest as false doctrine concerning the return of Christ. As we near the sudden return of Christ for His church, that "mystery of iniquity" will abound ever greater and will be seen in many, many false doctrines taking root
within Christendom.
The world has been growing more wicked, and now we see the world's doctrines being incorporated into church doctrines and taught in pulpits (Psychology, evolution, higher criticism, homosexual rights, ecumenism, etc.). It was not long ago, that these doctrines of the world were foreign to the church, but now they have been sneaked in by men who have "crept in unawares" bringing damnable heresies... and when sound men stand up to oppose them, they are mocked as "close-minded" or "old fashioned" etc.
This is happening even in the Baptist churches... and more so each day.
[Gets off soapbox]