Little children, it is the last hour; and as you have heard that the Antichrist is coming, even now many antichrists have come, by which we know that it is the last hour. They went out from us, but they were not of us; for if they had been of us, they would have continued with us; but they went out that they might be made manifest, that none of them were of us. But you have an anointing from the Holy One, and you know all things. I have not written to you because you do not know the truth, but because you know it, and that no lie is of the truth. Who is a liar but he who denies that Jesus is the Christ? He is the antichrist who denies the Father and the Son. (1Jo 2:18-22 NKJ)
John wrote his gospel, the letters and Revelation of Christ fifty plus years after the death of Christ. The Church was going through extreme persecution, especially under Nero and Domitian. Christians believed that the end was coming and that Jesus must return soon. John calls this the last hour. The rumor was that Nero, the Antichrist, was coming back to life, that Domitian may even be Nero incarnate.
John reminds the people and even us, that antichrist is anyone who, “denies that Jesus is the Christ? He is the antichrist who denies the Father and the Son.” Don’t just look outside, look inside as well.