Which Jesus?
There are many views about who Jesus is. The Qur'an (the holy book of Islam) has lofty things to say about Jesus being a prophet sent from God who taught with authority. Mormons believe in Jesus. And Jehovah's Witnesses claim to know who Jesus is. Many Americans assume that to be American is to be Christian, even though there is no discussion about who Jesus actually is.
We need to be able to tell the difference between what is true and what is false, because we need to know the truth in order to become alive!
In 1 John 4, there are clear indicators that allow us to do this. But this is not how many of us think today. Diversity reigns in contemporary culture. We assume that it is unloving to "judge" what someone believes as false. According to this way of thinking, there is no such thing as truth or falsehood; it's all about personal preferences. And anyone who claims to know truth in an absolute sense is ignorant, intolerant, and dangerous. However, this was not always assumed. According to the apostle John (one of Jesus' first followers), there are clear indicators about what it means to be "from God."
A clear indicator about what it means to be Christian is: What do we believe and confess about who Jesus is?
The 1st indicator of truth is about Jesus: What do we believe and confess about Jesus' identity? Who is Jesus? The 2nd indicator of truth is about the apostles (i.e. original followers of Jesus whom He gave authority to represent Him). Do we listen to the apostles, or do we listen to someone else? Here is what the apostles say about Jesus and those who claim to be "from God":
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. By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God, 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. Little children, you are from God and have overcome them, for He who is in you is greater than he who is in the world. They are from the world; therefore they speak from the world, and the world listens to them. We (the apostles) are from God. Whoever knows God listens to us; whoever is not from God does not listen to us. By this we know the Spirit of truth and the spirit of error....And we (the apostles) have seen and testify that the Father has sent His Son to be the Savior of the world. Whoever confesses that Jesus is the Son of God, God remains in Him and he in God. (1 John 4:1-6, 14-15) (italics, my addition)
The apostles claim that Jesus is "the Son of God" who "has come in the flesh." Do we listen to what the apostles say about Jesus, or do we believe other claims about who Jesus is? This is how we know if someone is "from God" or not. What do we believe about Jesus' identity?
Is Jesus really God?
The Christian claim - that Jesus is God - is crazy. If someone claimed to be an alien, would you believe him? Of course not! You would say he's insane, and you would be right to do so. Then why do we believe Jesus' claim to be God? Why do we listen to his followers who insist that he is God?
Jehovah's Witnesses say that Jesus is not really God. However, they do hold Jesus in high regard and say that he is preeminent as the first being ever to be created. Jesus is not the Creator; he is only a creature, but he does have an important status over all other created beings. On the other hand, Mormons claim that Jesus is the same as the Father. However, they do not believe that the Father is God in the same way that Christians do (i.e. Creator of all things and perfect in every way).
Is Jesus really human?
Some say that Jesus isn't really human; he just appeared to be. According to this view, Jesus is the Son of God (i.e. deity) who came for our salvation, but he didn't actually have a physical body. Some believe that Jesus did not become human because if he did so he would have corrupted himself (being God) by taking on a physical body. Others say that Jesus was in fact human in the sense that he had a physical body, but he didn't have the immaterial parts of humanity (i.e. mind, soul, spirit).
Popular opinion suggests that Jesus was only human.
Muslims claim that Jesus was a great prophet, and their holy book (the Qur'an) has more to say about Jesus than most of them realize. However, they emphatically deny that He is God. They have a high view of God. They believe (along with Jews) that there is only one God, so the claim that Jesus is the Son of God is incomprehensible and blasphemous to them.
Many Americans have been influenced by the beliefs of some of America's Founding Fathers, represented by Thomas Jefferson in this regard. These important figures of American history rejected anything supernatural in the Bible because it was contradictory to reason and science. They did not believe that Jesus actually did any of the miracles recorded in the Bible and so they did not believe that He was actually God. Therefore, they rejected the Christian belief that Jesus is the God-man - the eternal Son of God who became human.
Christians have always believed that Jesus is the God-man. But why is it so important to believe that Jesus is both fully God and fully man? Why does it matter what we believe about Jesus' identity?
Jesus' identity as the God-man is central to the gospel itself. God cares so much about our humanity that He humbled Himself by becoming human for us. So this tells us that God cares deeply about our humanity (body, mind, soul: everything "human" about us), so much so that He actually did something about it - He became human in order to redeem our humanity. Since Jesus is the God-man, God is not far away. He is near. He understands our pain and suffering. In the person of Jesus Christ, God is transforming our humanity from the inside out.
Our belief that Jesus is the God-man shows us that central to God's ways is to love at great cost. In the person of Jesus Christ, God has loved us at great cost to Himself. So to claim to have fellowship with God, without having this kind of sacrificial love, doesn't make sense. In fact, it is denying the very gospel with our actions. Jesus' identity as the God-man shows us who God really is: the One who loves with radical self-sacrifice.