Beloved, do not believe every spirit, but test the spirits to see whether they are from God, for many false prophets have gone out into the world. 2 By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God, 3 and every spirit that does not confess Jesus is not from God. This is the spirit of the antichrist, which you heard was coming and now is in the world already. ~~1 John 4:1-3 (ESV)Before I offer my thoughts on this passage, I really want to clarify that agreement with the Incarnation isn't the only criteria for discernment. 1 John 4:6, for instance, elaborates that false prophets don't rely on the apostles' teaching as being God's Word, and 2 Peter 2:18-19 tells us that they use sensuality to attract followers. I could go on, but I really just wanted to make the point that 1 John 4:1-3 gives us only one of several tests for evaluating spiritual teachers and their messages.
That said, the apostle John insists that belief in God coming as Man is essential Christian teaching. Right away, his plumbline eliminates groups like the Mormons and Jehovah's Witnesses from consideration as viable Christians. But I see something even deeper in this passage.
First, we must recognize Christ's divinity. We do so, not merely by verbally confessing an intellectual assent that He is God (although such assent must obviously occur), but by demonstrating faith in His deity by obeying His commands. Do we accept His authority, or do we twist and cherry-pick His Scripture to suit our own agendas? Believing that Jesus is God necessitates that we obey Him.
Secondly, we must recognize His humanity. If He had merely appeared as a Man, as He occasionally did in the Old Testament (please excuse me from citing references here, but I'm having a hard enough time typing today without that extra work), His death on the cross wouldn't have been an actual sacrifice. Therefore, any denial that He lived and died as a flesh-and-blood Man undermines the very heart of the Gospel.
That birth that we celebrated yesterday means so much to the Christian narrative, and any corruption of its meaning distorts the Christian faith. Even subtle denials of His Incarnation can spiral into heresy that could plummet us into eternal damnation. Thankfully, God's Word reveals Jesus in His full deity and His full humanity, allowing us to know the truth. We can praise Him that, in giving us the gift of Himself, He also gave us the gift of distinguishing truth from error.