What are the arguments of those who believe that the prerequisite stated here by John is yet not sufficient for being born of God? 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.
We must be always cautious when we remove a sentence from its context, because it can often lead to misunderstanding.  The verse you quoted was in the context of an entire letter.  It was not intended to be separated from that context.  Authorial intent is a critical consideration.

Furthermore, the audience should be considered as well.  The people to whom John wrote the letter believed certain things about Jesus, so He wasn't addressing misconceptions or heresies about who Jesus is.  They didn't have those misconceptions or heresies, while some who read that letter today certainly do.  It is important to note that Scripture can never mean what it was never intended to mean.

If we do divorce this verse from the context and the audience, we would have to conclude that someone who believes that Jesus is the Christ, but who didn't die on the cross, was not the Son of God, wasn't resurrected, wasn't sinless, and that salvation comes through the works of the law, and that adultery and lying and cheating are alright for him, would still be considered born of God.  Is it rational to assume that the Apostle John, if asked for clarification, would have agreed with this?  Of course not.  This is absurd, of course, and is an improper manner in which to treat Scripture.

Those that read this letter from John originally understood his intent.  Had they been believing heresy, like the Galatians, John would certainly have taken a different tone altogether.

So, from the perspective of a Biblical literalist, everyone who believes that Jesus is the Christ, the Son of God, co-equal and co-eternal with the Father and Spirit, who died on the Cross the make atonement for our sins, and who rose from the dead, and trusts the work of Jesus for salvation and forgiveness, is born of God.