1 John 4:3 reads, “And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world.” The chapter is about determining which spirits are of Christ and which are of the antichrist. The point I want to make is, the spirit of the antichrist is already in the world.
Matthew 24:12 tells us aniquity will abound. The spirit of the antichrist was already in the world at the time John wrote his letter. Aniquity already existed. The difference is, it will be more prevelant and less hidden.
I’m also contesting the theory that the first horseman is the antichrist entering the world. The spirit has been here for thousands of years. One could argue that the spirit of the antichrist manifests in the flesh with the first horseman. However, I want to point out the horsemen themselves could be spirits. (compare Revelation 6 to Zachariah 6).