By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God, and every spirit that does not confess Jesus is not from God. This is the spirit of the antichrist, which you heard was coming and now is in the world already. (1 John 4:2-3) Is John saying that the antichrist was already present at the time of this book’s writing?

No. Notice he wrote that ‘every spirit that confesses that Jesus Christ has come in the flesh is from God.’ He then says that ‘every spirit that does not confess Jesus is not from God. This is the spirit of the antichrist which you heard was coming and now is in the world already.’ Obviously there was people who did not confess that Jesus Christ had come in the flesh; the Christians were being hunted by them! These people had the spirit (also translated attitude  or disposition) of the antichrist. So the real question should be what or who was the antichrist? The answer to that is anyone who opposes Christ. They have been around since the time of Christ and they are still around today.