In John 3:18 Jesus said “but he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God.”
John 8:42 Jesus said unto them,If God were your Father, ye would love me: for I proceeded forth and came from God; neither came I of myself, but he sent me. 8:43 Why do ye not understand my speech? even because ye cannot hear my word. 8:44 Ye are of your father the devil, and the lusts of your father ye will do. He was a murderer from the beginning, and abode not in the truth, because there is no truth in him. When he speaketh a lie, he speaketh of his own: for he is a liar, and the father of it. 8:41 Ye do the deeds of your father. Then said they to him, We be not born of fornication; we have one Father, even God.
I John 3:10 In this the children of God are manifest, and the children of the devil: whosoever doeth not righteousness is not of God, neither he that loveth not his brother.
So before we believe we are condemned to death and hell and are the children of the devil. This is because of sin. Sin separates a person from God. As Paul says in Eph 2:1 and 5, we “were dead in trespasses and sins” and “having no hope, and without God in the world” (verse 12). But the good news is that God loved us so much that Jesus died on the cross to forgive us our sin. In John 1:12 the Bible says “But as many as received him, to them gave he power to become the sons of God, even to them that believe on his name.” So when we receive Jesus, which means when we believe on Him, we are adopted and become the children of God. As Paul says in Eph 1: 5-6, Having predestinated us unto the adoption of children by Jesus Christ to himself, according to the good pleasure of his will, To the praise of the glory of his grace, wherein he hath made us accepted in the beloved. 1 John 3:1-2 Behold what manner of love the Father has bestowed on us, that we should be called children of God! Therefore the world does not know us, because it did not know Him. 2 Beloved, now we are children of God; and it has not yet been revealed what we shall be, but we know that when He is revealed, we shall be like Him, for we shall see Him as He is.
One objection that some people raise to this is Acts 17.28 “For in him we live, and move, and have our being; as certain also of your own poets have said, For we are also his offspring.” Some people think that offspring here means children, and so think that all are children of God, believers and unbelievers. We can only find two Bible versions, the NASB and the Expanded Bible, which translate the word “children” here. But they are wrong – this would contradict all the verses we have just quoted, which make it very clear that not all are the children of God. Not one other Bible version uses the word children. I have three Greek Bible dictionaries including Vines and Strongs and all of them agree its the Greek word “genos” and don’t even mention children as a translation of the word, as it does not mean children. Teknon or huios are the usual words for child not genos. Genos means race, in the sense that of verse 26, that he has made all people from one blood, there is one human race. Not that we are all children of God. We are all sinners and condemned, children of the devil until we believe. Then we are adopted as children of God and receive eternal life. What a wonderful simple gospel.