Whoever denies the Father and the Son, this is the antichrist.
As St. John explains it, the antichrist is not a supernatural being with powers like one of the x-men. It is a regular human being like you or me. In Monday's reading, he even used the term in the plural,
so now many antichrists have appeared.
Thus we know this is the last hour.
They went out from us, but they were not really of our number;
if they had been, they would have remained with us.
Clearly they were members of the community, but members who, as St. John describes it, never really bought into the message of Jesus Christ, and ultimately ended up leaving the Church.
While the supernatural movie version is more entertaining and, perhaps, more interesting, what St. John is talking about is much more applicable to our daily lives. If we're not careful any one of us could become the antichrist. While we may not come right out and deny Jesus Christ, we can if we are not careful deny Christ by our actions. We can become complacent and accept sin as if it were not sin. This is what gives scandal, when people see Christians apparently not even trying to be better.
The Pope addressed this when he quite directly said,
How many times have we witnessed the scandal of those who go to church and spend all day there or attend every day, and later go on hating others or speaking ill of people. This is a scandal,...It would be better to not go to church. Live like an atheist.
If you go to church, then live like a son, like a brother, like an authentic witness, not a counter-witness.
All of us fall short of living the fullness of the gospel. We give into the passions and temptations of earthly life. But when we do, we must acknowledge our sin, go to the Sacrament of Penance, and strive each day to more fully live the gospel. Tomorrow should always be better than today.