Whosoever believeth that Jesus is the Christ is born of God (1st John 5:1)
John seems to be both liberal and absolutistic in his statement here: on one hand, he doesn't add any more prerequisites for a human to be a regenerate (while today's main stream Christianity would most likely add some more like believing that Christ is God, that Christ and God the Father are the same One God, etc.), on the other hand, he says "whosoever", which sounds like he is very explicit about his statement being true for all cases that meet his prerequisite (which is believing that Jesus is Christ).
What are the arguments of those who believe that the prerequisite stated here by John is yet not sufficient for being born of God? I know that many Christians don't consider, for example, Mormons and Jehovah's Witnesses to be regenerate (because they don't believe that Jesus is the same God as God the Father). However, both Mormons and Jehovah's Witnesses seem to meet this prerequisite stated here by John.