Antichrists exist today. "Little children, it is the last time: and as ye have heard that antichrist shall come, even now are there many antichrists; whereby we know that it is the last time" (1 John 2:18).
And they were already showing themselves back them, here is something I came across. Take a look at 2 Thessalonians 2:7.
2 Thessalonians 2:7 King James Version (KJV)
For the mystery of iniquity doth already work: only he who now letteth will let, until he be taken out of the way.
2 Thessalonians 2:7 (New International Version)
7For the secret power of lawlessness is already at work; but the one who now holds it back will continue to do so till he is taken out of the way.
2 Thessalonians 2:7 (New Living Translation)
7 For this lawlessness is already at work secretly, and it will remain secret until the one who is holding it back steps out of the way
So the mystery of iniquity or lawlessness was already at work secretly, and what is holding it back. Well lets start with the power holding it back:
For more than 500 years, half a millennium, Roman Empire was invincible, but it too finally faded and it stopped holding back what prophecy had foretold.
Here is the Geneva Study Bible:
(7) He foretells that when the empire of Rome is taken away, the seat that falls away from God will succeed and hold its place, as the old writers, Tertullian, Chrysostom, and Jerome explain and interpret it... He who is now in authority and rules all, that is, the Roman Empire
Wesley's Explanatory Notes
..This mystery of iniquity is not wholly confined to the Romish church, but extends itself to others also. It seems to consist of, Human inventions added to the written word. Mere outside performances put in the room of faith and love. Other mediators besides the man Christ Jesus. The two last branches, together with idolatry and bloodshed, are the direct consequences of the former; namely, the adding to the word of God. Already worketh - In the church. Only he that restraineth - That is, the potentate who successively has Rome in his power. The emperors, heathen or Christian; the kings, Goths or Lombards; the Carolingian or German emperors.
Jerome (347-420): "Says the apostle [Paul in the second epistle to the Thessalonians], 'Unless the Roman Empire should first be desolated, and antichrist proceed, Christ will not come.'
So it is a antichrist power that the Roman Empire was holding back, so what is this mystery of iniquity:
Revelation 17:5,9
5And upon her forehead was a name written, MYSTERY, BABYLON THE GREAT, THE MOTHER OF HARLOTS AND ABOMINATIONS OF THE EARTH.
9And here is the mind which hath wisdom. The seven heads are seven mountains, on which the woman sitteth.
In the day of the Apostle John, the city of Rome was famed for its situation of encompassing seven hills, Capitolinus, Palatinus, Aventinus, Esquilinus, Coelius, Viminalis, and Quirinalis, as noticed by Virgil, Horace, Ovid, Claudian, Starius, Martial, and others.
The mystery of Babylon had been a power which brought corruption of man and lawlessness. It had been known more by what it did to deceive and substitute itself above God, but Paul now begins to point it out and we see it is at the city of Rome after the Roman Empire had stepped out of the way.