this was [the spirit] of antichrists [plural]that wentout, many means more then one.
this man antichrist that will come later is not the same here
which you have heard was coming, and is now [already in] the world (I John 4:3 ).
Little children, it is the last hour; and as you have heard that [the Antichrist] is coming[future],
[even now [many] antichrists] have come-----
this happened way back then, the church was taken over from within,
Paul cried for 3 years with tears because he knew this.
The horseman [has] wreaked havoc on mankind with false religion,
then followed war throughout history.
this man antichrist that will come later is not the same here
which you have heard was coming, and is now [already in] the world (I John 4:3 ).
Little children, it is the last hour; and as you have heard that [the Antichrist] is coming[future],
[even now [many] antichrists] have come-----
this happened way back then, the church was taken over from within,
Paul cried for 3 years with tears because he knew this.
The horseman [has] wreaked havoc on mankind with false religion,
then followed war throughout history.
Apostle John's concept of "antichrist" is given in the singular tense, and in the plural tense with "many antichrists". It's because of the subject of the "mystery of iniquity" that Apostle Paul mentioned in 2 Thess.2 while also warning us about that specific false one that will sit in the temple of God, showing himself that he is God, and exalting himself over all that is called God, or that is worshiped.
In reality per Scripture, that's TWO objects, a singular false one, and then the many false ones.
Our Lord Jesus covered the same idea in His Olivet Discourse. In Matt.24:5 He warned us about the idea of many who come in His name saying they are Christ. That's the "many antichrists" of John (1 John 2:18).
But in the Matt.24:23-26 verses, our Lord Jesus is warning us of a specific false one, the one that's to come to sit in the temple that Paul taught in 2 Thess.2.
Matt 24:21-26
21 For then shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be.
22 And except those days should be shortened, there should no flesh be saved: but for the elect's sake those days shall be shortened.
23 Then if any man shall say unto you, "Lo, here is Christ, or there"; believe it not.
24 For there shall arise false Christs, and false prophets, and shall shew great signs and wonders; insomuch that, if it were possible, they shall deceive the very elect.
25 Behold, I have told you before.
26 Wherefore if they shall say unto you, "Behold, he is in the desert"; go not forth: "behold, he is in the secret chambers"; believe it not.
KJV
Those parts in Red are about specific false messiah that is to come, the one of 2 Thess.2:3-4 that Jesus will destroy with the brightness of His coming. The KJV translators translated Greek pseudochristos to "false Christs" there, when the warnings just before it and right after it are in the singular tense (the parts in red). This is why Dr. James Strong in his Strong's Exhaustive Concordance has the translation of pseudochristos as 'a spurious Messiah' (Strong's no.5580), pointing to a singular false messiah.
So what was already... at work in the world? The "many antichrists" which is what Paul's "mystery of iniquity" is about, and also what our Lord Jesus' warning about many false prophets coming in His name saying they are Christ. We have already had many examples of that working in our time, fakes who say they are Christ.
But the one of Matt.24:23-26 and 2 Thess.2:4, which is also the same "another beast" of Rev.13:11 forward, we have NOT seen that one appear on earth yet, for he is to come at the end during the tribulation, and is that beast king of Rev.17, the "vile person" of Dan.11, and the "little horn" of Dan.8. It is he that the ten horns (ten kings) of Rev.17 will give their power to for "one hour".