NOT ALL OF US ARE GOD'S CHILDREN

You've heard it said, "We are all God's children." This rings flowery and nice. It's an insidious lie.
Indeed, God both created and loves – in a way most unfathomable – everyone who ever lived. He wove us together in our mother's womb and numbered our every hair. But God the Father has only one begotten Son. The rest of us, in order to become one of God's children, must be adopted – in, by and through – the One Who is the Son: Jesus Christ. Those who are not adopted are not children of God. Christ and Christ alone is "the way, the truth and the life."
Indeed, to become a child of God, we must ask God – through Christ – to adopt us. We mustn't just believe upon Him – for "Even the demons believe that" (see James 2:19) – but, rather, we must also accept Him. We must follow Jesus, the one true God, as our only God. "But to all who believed him and accepted him (Jesus), he gave the right to become children of God." (John 1:12)
Don't believe? Don't accept? You have no right to become a child of God.
And that's unholy hell.
I'm not here to question God. I can neither fully understand nor explain why what He says is so. I can only convey to you that He unmistakably, unequivocally and without stuttering, says it is so. (Source:ChristianPost)

Question: "Are we all God's children, or only Christians?"

Answer: The Bible is clear that all people are God’s creation (Colossians 1:16), and that God loves the entire world (John 3:16), but only those who are born again are children of God (John 1:12; 11:52; Romans 8:16; 1 John 3:1-10).

In Scripture, the lost are never referred to as God’s children. Ephesians 2:3 tells us that before we were saved we were “by nature objects of wrath” (Ephesians 2:1-3). Romans 9:8 says that “it is not the natural children who are God's children, but it is the children of the promise who are regarded as Abraham's offspring.” Instead of being born as God’s children, we are born in sin, which separates us from God and aligns us with Satan as God’s enemy (James 4:4; 1 John 3:8). Jesus said, “If God were your Father, you would love me, for I came from God and now am here. I have not come on my own; but he sent me” (John 8:42). Then a few verses later in John 8:44, Jesus told the Pharisees that they “belong to your father, the devil, and you want to carry out your father's desire.” The fact that those who are not saved are not children of God is also seen in 1 John 3:10: “This is how we know who the children of God are and who the children of the devil are: Anyone who does not do what is right is not a child of God; nor is anyone who does not love his brother.”

We become God’s children when we are saved because we are adopted into God’s family through our relationship with Jesus Christ (Galatians 4:5-6; Ephesians 1:5). This can be clearly seen in verses like Romans 8:14-17: “…because those who are led by the Spirit of God are sons of God. For you did not receive a spirit that makes you a slave again to fear, but you received the Spirit of sonship. And by him we cry, ‘Abba, Father.’ The Spirit himself testifies with our spirit that we are God's children. Now if we are children, then we are heirs—heirs of God and co-heirs with Christ, if indeed we share in his sufferings in order that we may also share in his glory.” Those who are saved are children “of God through faith in Christ Jesus” (Galatians 3:26) because God has “predestined us to be adopted as his sons through Jesus Christ, in accordance with his pleasure and will” (Ephesians 1:5). (Source:GotQuestion)