"You have heard" is a past tense verb. It was in the past that they heard that antichrist (there is no the in the greek) is coming. NOW many antichrists have appeared.
1 John 2:18 Children, it is the last hour; and just as you have heard that antichrist is coming, so now many antichrists have appeared. This is how we know it is the last hour
"You have heard" is a past tense verb. It was in the past that they heard the spirit of the antichrist is coming. and Now is already in the world.
1 John 4:3 and every spirit that does not confess Jesus is not from God. This is the spirit of the antichrist, which you have heard is coming, and is already in the world at this time.
An earthly example for understanding:
You have heard that coke vanilla orange is coming, so now coke vanilla orange has appeared all over.
1 John 2:18 Children, it is the last hour; and just as you have heard that antichrist is coming, so now many antichrists have appeared. This is how we know it is the last hour
"You have heard" is a past tense verb. It was in the past that they heard the spirit of the antichrist is coming. and Now is already in the world.
1 John 4:3 and every spirit that does not confess Jesus is not from God. This is the spirit of the antichrist, which you have heard is coming, and is already in the world at this time.
An earthly example for understanding:
You have heard that coke vanilla orange is coming, so now coke vanilla orange has appeared all over.