The term 'anti' means to be opposed to or against something or someone. We live in an era that is even more opposed to Christ than when He walked the earth.
18 Dear children, the last hour is here. You have heard that the Antichrist is coming, and already many such antichrists have appeared. From this we know that the last hour has come. 19 These people left our churches, but they never really belonged with us; otherwise they would have stayed with us. When they left, it proved that they did not belong with us. 20 But you are not like that, for the Holy One has given you his Spirit, and all of you know the truth (1 John 2:18-20, NLT)
If the apostle John could write that it was the last hour when he was still on earth, then today (roughly 2000 years later) it must be the last minutes. John said that the appearance of "many such antichrists" is evidence that we are in the last days.
John continued to reveal that many people who "oppose Christ" (I.e. antichrists) are actually those who used to be in our churches! However, the fact that they did leave, proves that they were never really true believers in the first place. The people who leave the church and come against Christ do not have the Spirit of Christ and therefore, they are not His. On the other hand, we do know the truth because we (followers of Christ) have the Spirit of a God living in us and He teaches us all things; what is true and what is right.