Mark 1:23-4 And there was in their synagogue a man with an unclean spirit; and he cried out, Saying, Let us alone; what have we to do with thee, thou Jesus of Nazareth? art thou come to destroy us? I know thee who thou art, the Holy One of God. (See also Luke 4:33.)
1 John 4:2 Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God...
Here, an unclean spirit confessing that Jesus is of God. According to 1 John 4, that same spirit must be of God, the Skeptics say.
In the Markan passage, the demon in the man is not "confessing" that Jesus Christ has come in the flesh so much as the demon is merely stating this fact. Is stating something the same as confessing? The objection is forcing these two words to be equivalent when there is no reason to.
Also, the context of 1 John 4 is different than that of Mark. John addresses his readers in verse 2: "This is how you [ie the receivers of this epistle] can recognize the Spirit of God....". The author of 1 John's exhortation to the readers of the epistle is giving members of the Church a way for them, mere humans that they are, to distinguish between the Spirit of God and lying spirits.
The Markan passage does not deal with anything of the sort. It merely reports an encounter Jesus had with a demon. As such, it is a category fallacy to compare these two passages.