Anyway, the "anyone" in the above paragraph includes everyone, past, present, and future.
And, if those who "left the group" were engaging in the behaviors in the above paragraph, then they were "antichrists", or, to make it easier, "against Christ".
Again. Who do you think John was speaking of that left their group and was spreading falsehoods?
1 John 2
18 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.
19 They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us.