At 2 Thessalonians 2, the apostle Paul wrote that "we ask you not to be quickly shaken from your reason nor to be alarmed either by an (so-called) inspired statement or by a spoken message or by a letter appearing to be from us, to the effect that the day of Jehovah is here. Let no one lead you astray in any way, because it will not come unless the apostasy comes first and the man of lawlessness gets revealed, the son of destruction. He stands in opposition and exalts himself above every so-called god or object of worship, so that he sits down in the temple of God, publicly showing himself to be a god. Do you not remember that when I was still with you, I used to tell you these things ?"(2 Thess 2:1-5; Note: the word "apostasy" is from the Greek verb aphistemi that means "stand away from", with the noun having the sense of "desertion; abandonment; rebellion")
Pagan Rome did not restrain apostasy, but rather it was a place where apostasy grew, not hindered. How many recognize that the "Holy Trinity" that the Catholic church espoused was absorbed from the trinity of pagan Rome, that of Jupiter (supreme god and protector of Rome), Juno (his consort) and Minerva (goddess presiding over handicrafts), that "formed the triad whose worship was the central cult of the Roman state" ?(2005 Microsoft Reference Library)
Note: the word "catholic" means "broad, wide-reaching, all-embracing", so that when the Catholic church came into existence near the end of the 4th century C.E. (or about 381 C.E. as Rome's official state religion under Roman Emperor Theodosius I who imposed Trinitarian "Christianity"), it embraced pagan Rome's teachings and practices, such as the Pontifex Maximus or Pontif, that was the "Supreme Priest" of the religions of Rome and in which Caesar Augustus became the first "Supreme Priest" or Pontifex Maximus in 12 B.C.E., and whereby the pope today is called "the Pontif ", so that Catholicism is "broad" and "wide-reaching" in acceptance of false teachings in order to draw in the people)
With adroit precision, French historian Henri Marrou (1904-1977 C.E.) wrote: “By the end of the reign of Theodosius (in 395 C.E.), Christianity, or to be more precise, orthodox Catholicism, became the official religion of the entire Roman world.”
Orthodox Catholicism had replaced true Christianity and had become a “part of the world.” This State religion was vastly different from the religion of Jesus’ early followers, to whom he said: “You are no part of the world.”(John 15:19)
What the apostle Paul wrote in his second letter to the Thessalonians in about 51 C.E. came to fruition at the end of the 4th century C.E., whereby total apostasy was now on full display with Catholicism, that later grew massively into Christendom, in which there came to be Catholics, Protestants, Episcopalians, Methodists, Church of Christ, Mormons, Baptists, Presbyterians, etc., so that there are now some 41,000 different sects and denominations in it.
So, at 2 Thessalonians 2:6, he spoke of "what is acting as a restraint" to the uprising apostasy that was already at work within the Christian congregation, and whereby at Acts 20, Paul told the Ephesian elders: "I know that after my going away oppressive wolves will enter in among you and will not treat the flock with tenderness, and from among you yourselves men will rise and speak twisted things to draw away the disciples after themselves."(Acts 20:39)
From within the Christian congregation, "oppressive wolves will enter in among you and not treat the flock (of genuine Christians) with tenderness", these being the first ones that make up the "anti-christs", for the apostle John wrote: "Young children, it is the last hour, and just as you have heard that the antichrist is coming, even now many antichrists have appeared, from which fact we know that it is the last hour (before the Christian congregation is overrun with counterfeit Christians). They went out from us (or came from within the Christian congregation), but they were not of our sort (wanting to promote false or apostate ideas, twisting Scripture); for if they had been of our sort, they would have remained with us. But they went out so that it might be shown that not all are of our sort."(1 John 2:18, 19)
It was these "anti-christs" (that means "of or instead of Christ", supplanting true Christian teachings with their own "brand" of teaching and calling it "Christian") who became Catholicism that grew into a massive Christendom, that is so dominant today.
The apostles fought to stem the tide of the initial apostasy while in its infancy, but after their deaths, as Jesus stated in his illustration at Matthew 13:24-30, that "while men were sleeping, his (Jesus) enemy came and oversowed weeds (counterfeit Christians) in among the wheat (genuine Christians) and left".(Matt 13:25)
Only in these "final part of the days" (that began in 1914), has there been a restoration of true Christianity, as seen at Isaiah 2:2-4, that says: "In the final part of the days (or before this "world" is "crushed out of existence, Dan 2:44), the mountain (or governmental sovereignty) of the house of Jehovah (or true worship that is exercised through God's heavenly Kingdom) will become firmly established above the top of the mountains (or political governments), and it will be raised up above the hills, and to it all the nations will stream. And many peoples will go and say: “Come, let us go up to the mountain of Jehovah, to the house of the God of Jacob. He will instruct us about his ways, and we will walk in his paths. ”For law will go out of (symbolic) Zion, and the word of Jehovah out of (symbolic) Jerusalem. He will render judgment among the nations and set matters straight respecting many peoples. They will beat their swords into plowshares and their spears into pruning shears. Nation will not lift up sword against nation, nor will they learn war anymore."