Well, Jesus does say to love our enemies (Matthew 5:44-47), and we can certainly honor and respect everyone we meet, but the New Testament writers warn us about false prophets (2 Peter 2:1; Jesus calls them “wolves in sheep’s clothing”—Matthew 7:15), antichrists (1 John 2:18), people trying to lead us astray (1 John 2:26), purveyors of a different gospel (Galatians 1:8), wolves who distort the truth (Acts 20:29), etc. And Jesus himself told us that if a brother doesn’t listen after being reproved, to “treat him as you would a pagan or a tax collector” (Matthew 18).
But wait; aren’t we all God’s children? Well—no, we aren’t; John writes that Jesus gave the power (or the right) to become children of God to those who welcome him, who believe in him (John 1:12). This would be meaningless if everyone were already God’s child. Jesus also said to some religious leaders in his day, “You are children of your father the devil” (John 8:44), and Paul writes about being adopted (Romans 8). Although every human being is created in God’s image, we are not all his children in the way Jesus or John or Paul thinks, until something happens.
What is that something? Well, the Bible doesn’t tell us in a scientific or systematic way; it’s not like the DSM or the Motor Vehicle Code. The biblical writers use a number of different terms: John 1:12 talks about receiving or welcoming Jesus, which is related somehow to believing in his name; those who do that can become children of God. Similarly John 3:14-15 talks about believing in Jesus, who would be lifted up the way the snake was lifted up in the desert; those who believe receive eternal life. The Apostle Paul talks about a righteousness from God, which comes through faith in Christ to all who believe (Romans 3:21-22) [mouse here for more].
So not everyone is God’s child; is it enough if someone says he believes in Christ? Well, what does he believe about Christ? When the Apostles talk about believing in Christ, they mean believing what they say about him—for example that he was a real person who died for our sins and rose from the dead. In his gospel and his first letter, John begins by talking about Jesus and the fact that he (John) personally saw him: “The word became flesh and dwelt among us... and we have seen his glory” (John 1:14); “That which was from the beginning, which we have heard, which we have seen with our eyes, which we have looked at and our hands have touched...” (1 John 1:1). John is also specific in his instructions about testing the spirits:
Dear friends, do not believe every spirit, but test the spirits to see whether they are from God, because many false prophets have gone out into the world. This is how you can recognize the Spirit of God: Every spirit that acknowledges that Jesus Christ has come in the flesh is from God… (1 John 4:1-2)I take the phrase “has come in the flesh” as shorthand for “has come in the flesh, dwelt among us, taught us about God, died for our sins, and was raised from the dead”—as Paul says in the first few lines of 1 Corinthians 15.
At least this is how I read the New Testament writers. I don’t think someone has to believe in the historicity of Jonah or Hosea to be a brother in Christ; they don’t have to agree with me on the rite of baptism, they needn’t share my views on sexuality, politics, fiscal or monetary policy, or women’s ordination to be a brother in Christ. They could be a communist or a racist or a libertarian, they might have bad table manners, they might pick their nose or scratch their armpits in public, they might smoke tobacco or marijuana; if they believe Christ died for their sins, if they welcome him into their lives, then they’re a brother (or sister) in Christ. We may choose our friends; we don’t choose our family members.
On the other hand, if someone doesn’t believe Jesus was a real person, or they think Jesus didn’t really suffer and die for our sins, or they don’t believe Jesus really rose from the dead, then they’re not a brother in Christ, though they may be a very nice person. Someone who thinks Jesus Christ was a kind of ideal concept, or who thinks the resurrection was some ahistorical tale, may be a great humanitarian, but they’re not in the family of God. They might go to the same church, they might dress nicely and speak politely and have the “right” views on a lot of social issues, but they’re neighbors or friends, not part of God’s family.