The end of the first century witnessed many heretical teachings. One of these heresies, Gnosticism, taught that Jesus Christ was not really a flesh-and-blood human being but a spirit that was manifested as a human being. This was undoubtedly one of the things John was alluding to when he wrote these verses.
However, there is also a deeper meaning to these words that John was inspired to write. The Holy Spirit inspired John to use the Greek perfect participle for the words "has come" in the above verses. The perfect tense implies not only the historical fact of Jesus Christ having been born as a flesh-and-blood human being but also the present continuance of this fact. John is saying that Jesus Christ is still human in the sense that He is living His life over again in human beings who submit to Him through the power of the Holy Spirit.
The message of this scripture is simply this: A teacher is of God if he teaches that Jesus Christ is coming—living His life over again in the flesh of every true, regenerated Christian—and that a Christian must follow Him wherever He leads and emulate Him in every way. But a teacher who teaches that one does not have to follow Christ and that it is not necessary for Christ to live in the flesh of His disciples is not of God. John says that this false teaching stems from the spirit of antichrist (verse 3).