Not everyone is a child of God, at least not in any sense that most people mean when they say such things. It is very common to hear people claim, "we are all God's children." By this, they generally mean that God has the same familial relationship with and parental obligation to every human being equally. Everyone is guaranteed a positive connection with God and even a birthright of sorts by which God is obligated to bless them in certain ways as their natural-born, loving Father. Some groups, like Mormonism, even go so far as to claim that we are God's children in the most literal sense. They teach that God procreated with his wife (or wives) in heaven and we are the result of that procreation. We are, they assert, the actual biological offspring of God; His literal children and the same species or type of being as He is. None of these claims are biblical or true.
Creation and Fatherhood
God is the creator of all things, including us. In a figurative way, we can call Him the "Father of creation" in the same sense that Henry Ford is the "father of the automotive industry" or that George Washington and John Adams are "founding fathers of the United States." But this is not a relational, familial, or literal sense. Acts 17:28-29, for example, says that all men are "God's children" specifically in that God "made the world and all things in it," (Acts 17:24) and that "He made from one man every nation of mankind to live on all the face of the earth," (Acts 17:26). Paul's argument here is not that humans are literally God's biological offspring or even that we have a special relationship with God like a father does with a child. Paul's only point is that "we ought not to think that the Divine Nature is like gold or silver or stone, an image formed by the art and thought of man," (Acts 17:29). Since we are a thing that God created, we ought not to think of God as something that we can create. Since God formed us, it is foolish to worship something we ourselves can form. Paul points back to the creation of men (as well as the heavens, the earth, and everything in them) and argues from this that idolatry is foolish, even sinful. This passage, however, does not identify God as the Father of humanity in any sense beyond that of the rest of creation. God is elsewhere called the "Father" of the lights in the heavens (James 1:17), yet no one thinks this means that the sun, moon, or stars are literally God's offspring or that they have a special family relationship with God. They are His creations. He made them, and therefore can be called, in a sense, their "Father," but not in a biological, reproductive, familial, or relational sense. The word "father" in such contexts is a figurative reference to God as their (and our) maker.
Paul and God's Adopted Children
Paul often opens his letters to the churches with phrases like "God our Father" (Romans 1:7, 1 Corinthians 1:3, etc), but these phrases apply to believers, not to all humanity. Indeed, Paul explicitly teaches that we become God's children by adoption. God lovingly takes in those who are in Christ as His children even though we are not rightfully His children at birth.
"But when the fullness of the time came, God sent forth His Son, born of a woman, born under the Law, so that He might redeem those who were under the Law, that we might receive the adoption as sons," (Galatians 4:4-5).
"He predestined us to adoption as sons through Jesus Christ to Himself, according to the kind intention of His will," (Ephesians 1:5).
The idea that individuals become children of God by the grace of adoption rather than by right of birth is also repeated in Paul's letter to the Romans (Romans 8:15-23, 9:4-8). He elsewhere explains to congregations of Christians:
"For you are all sons of God through faith in Christ Jesus. For all of you who were baptized into Christ have clothed yourselves with Christ. There is neither Jew nor Greek, there is neither slave nor free man, there is neither male nor female; for you are all one in Christ Jesus," (Galatians 3:26-28).
God took men of the Jewish and gentile nations and made from them one new people in Christ. Those are the people whom God has adopted. They have become His children by a gracious act of redemption. They were not born God's children and had no special right to be such. Paul carefully distinguishes between the children of God and the unbelieving world:
"Do all things without grumbling or disputing; so that you will prove yourselves to be blameless and innocent, children of God above reproach in the midst of a crooked and perverse generation, among whom you appear as lights in the world, holding fast the word of life, so that in the day of Christ I will have reason to glory because I did not run in vain nor toil in vain," (Philippians 2:14-16).
You were not born a child of God. You only become one by God's grace through faith in Christ Jesus. You enter to the family of God by adoption, not by natural birth.
John and Becoming God's Child
John expresses the same idea when he writes:
"He came to His own, and those who were His own did not receive Him. But as many as received Him, to them He gave the right to become children of God, even to those who believe in His name, who were born, not of blood nor of the will of the flesh nor of the will of man, but of God," (John 1:11-13).
John also explains:
"See how great a love the Father has bestowed on us, that we would be called children of God; and such we are. For this reason the world does not know us, because it did not know Him.," (1 John 3:1).
John makes a distinction between the world and the Children of God. Not all men are God's children, only those He has made His own by grace. John records Jesus' own words to those who claimed to be God's children by did not believe in Jesus:
"If God were your Father, you would love Me, for I proceeded forth and have come from God, for I have not even come on My own initiative, but He sent Me. Why do you not understand what I am saying? It is because you cannot hear My word. You are of your father the devil, and you want to do the desires of your father," (John 8:42).
Those who rejected Christ did so because they were not God's children, but the children of the devil. These words may sound harsh, but they are the very words of Christ. We cannot ignore them. By nature, by birth, we are children of Satan. We are rebellious and lost citizen's of the sinful and dying world with familial connections only to the evil one. We are not natural born members of the household of God. John likewise explains:
"By this the children of God and the children of the devil are obvious: anyone who does not practice righteousness is not of God, nor the one who does not love his brother," (1 John 3:10).
The children of God are a distinct people, not the whole of mankind. We also see in John's gospel:
"Now he did not say this on his own initiative, but being high priest that year, he prophesied that Jesus was going to die for the nation, and not for the nation only, but in order that He might also gather together into one the children of God who are scattered abroad," (John 11:51-52).
This parallels closely with Jesus' earlier words about those who are and are not His sheep. He explains His role in gathering the scattered flocks that belong to Him:
"I am the good shepherd, and I know My own and My own know Me, even as the Father knows Me and I know the Father; and I lay down My life for the sheep. I have other sheep, which are not of this fold; I must bring them also, and they will hear My voice; and they will become one flock with one shepherd," (John 10:14-16).
And yet also explains that there are those who reject Him precisely because they are not His sheep:
"You do not believe because you are not of My sheep. My sheep hear My voice, and I know them, and they follow Me; and I give eternal life to them, and they will never perish; and no one will snatch them out of My hand," (John 10:26-28).
Even in John 20:17, where Jesus speaks to Mary Magdalene of "My Father and your Father," He is speaking to a believer, one of His faithful followers. This by no means implies that God is everyone's Father. John clearly draws a line between those who have become children of God through Christ and those who remain in the world and are not God's children. This is identical to Paul's teaching that God makes believers His children by adoption.
The Covenant of Israel and God's Fatherhood
Just as the New Testament speaks of those who have come into fellowship with God through Christ as God's adopted children, the Old Testament used father/child language of God's covenant relationship to Israel. God told Moses to declare to Pharaoh that "Israel is my son," (Exodus 4:22) and to Jeremiah said, "I am a father to Israel," (Jeremiah 31:9). Israel were slaves in Egypt, but God set His special, fatherly love on them. He redeemed them and made them His own.
"You are a holy people to the Lord your God; the Lord your God has chosen you to be a people for His own possession out of all the peoples who are on the face of the earth. The Lord did not set His love on you nor choose you because you were more in number than any of the peoples, for you were the fewest of all peoples, but because the Lord loved you and kept the oath which He swore to your forefathers, the Lord brought you out by a mighty hand and redeemed you from the house of slavery, from the hand of Pharaoh king of Egypt," (Deuteronomy 7:6-8).
When Israel went astray, God said to Ezekiel:
"Then He said to me, 'Son of man, I am sending you to the sons of Israel, to a rebellious people who have rebelled against Me; they and their fathers have transgressed against Me to this very day. I am sending you to them who are stubborn and obstinate children, and you shall say to them, ‘Thus says the Lord God.’" (Ezekiel 2:3-4).
Through the prophet Malachi, God reminds Israel how He has destroyed other nations but has always restored Israel because of His covenant love for them. He then says to them:
"A son honors his father, and a servant his master. Then if I am a father, where is My honor? And if I am a master, where is My respect?’ says the Lord of hosts to you, O priests who despise My name," (Malachi 1:6).
In the same way, Malachi goes on to write:
"Do we not all have one father? Has not one God created us? Why do we deal treacherously each against his brother so as to profane the covenant of our fathers?" (Malachi 2:10).
Some people appeal to this verse to claim that Malachi is saying that God is a father to every human being He created. This could not be further from the truth. When Malachi speaks of the brotherhood of his readers and the Fatherhood of God over them all, he is speaking to them uniquely as Israelites distinct from the nations. God created Israel as a nation and people unto Himself. This is the creation to which Malachi appeals here. Notice the very next verses:
"Judah has dealt treacherously, and an abomination has been committed in Israel and in Jerusalem; for Judah has profaned the sanctuary of the Lord which He loves and has married the daughter of a foreign god. As for the man who does this, may the Lord cut off from the tents of Jacob everyone who awakes and answers, or who presents an offering to the Lord of hosts," (Malachi 2:11-12).
To the same people whom Malachi writes "Do we not all have one father? Has not one God created us?" he also immediately rebukes them for marrying "the daughter of a foreign god." People outside the covenant of Israel were not God's children. They were children of the false gods whom they served. Far from teaching the universal fatherhood of God, Malachi actually teaches that God is Father only to those with whom He has voluntarily entered into that relationship. Those in covenant fellowship with God are adopted as His children. Others are not God's children. Most of the time people ignore this distinction in reading the Old Testament and draw flawed conclusions by reading every human being into verses that simply don't apply to them.
Even in the New Testament, in places like Matthew 23:9 where Jesus is addressing not only His disciples but the larger crowds and still says "Do not call anyone on earth your father; for One is your Father, He who is in heaven." Some point out, "you see! Jesus was talking to the crowds and told them all that God was their Father!" We have to remember, however, that this was a crowd of Jews. There was a covenantal sense in which they understood God as the Father of their nation. We also have to remember that Jesus isn't telling them to deny their birth parents. This section is a rebuke on the scribes and Pharisees. People are not to call such religious leaders their spiritual fathers, rabbis, or lords. "Father" here is figurative. In no way does this passage imply that God is the literal or relational father of every human being everywhere.
While we all have one maker to whom we owe our gratitude, allegiance, and exclusive worship, God is not inherently the Father of every man and woman. We are not His literal, biological children and we are not guaranteed any special relationship to Him merely by right of birth. If anything, our sinful hearts make us, by nature, children of wrath, of our false gods, and indeed, of the devil. It is only by God's grace through the redemptive work of Jesus Christ received by faith alone that we are blessed with the opportunity to become children of God by adoption.