One of these mainstream concepts is the idea that God is our Father and, in the words of the vaguely religious song "The Prayer," that "we [i.e., human beings] are all God's children" (though some prefer thinking of God as "Parent" rather than "Father"—remind me to write a post on why this isn't satisfactory). This concept has become so pervasive that to suggest that people who don't belong to your religion aren't children of God is considered to be about as grossly hateful and intolerant a thing you can say.
But is the idea that "we are all God's children" biblical? As a matter of fact, it is! Well, sort of. Actually the Bible talks about God as a father and humans as his children in a variety of ways depending upon what the writer wants to convey at the time. Some of these ways are more inclusive than others. Here's a brief survey.
1. God is the Father of Israel. It was very uncommon in ancient Israel to call God "Father"—in fact, no individual ever considered him his own personal Father. But on rare occasions Israel collectively is portrayed as God's son, because God "begot" the nation of Israel into existence and maintained an affectionate, protective, providing, disciplining yet merciful relationship with Israel through its history, a relationship that he did not maintain with any other nation. (See Deut. 32:6; Isa. 63:16; 64:8; Jer. 3:19; Hos. 11:1; Mal. 2:10.)
2. God is the Father of God the Son. Overwhelmingly the most common attribution of Fatherhood to God pertains to his Fatherhood of Jesus Christ, his Eternal Word [Logos]. Think how often in the New Testament that we see statements like Colossians 1:3, "We always give thanks to God, the Father of our Lord Jesus Christ." John stresses that Jesus is God's "unique" (Greek monogenēs, "one of a kind," "one and only," in older [mis-]translations "only-begotten") Son—no one else has God for their Father like Jesus the Son of God does (John 1:14, 18; 3:16, 18; 1 John 4:9). That's because the Father's fatherhood of the Son is more than a figure of speech: the Son really (and from eternity past) comes from the Father and derives his existence from him, and he really is of utterly the same nature as him (see, e.g., Heb. 1).
But Jesus also claims God for his Father uniquely because Jesus is the True Israel, the one who in his life fulfilled all the covenant faithfulness that Israel was supposed to but fell short. So for example, as God says in Hosea 11:1, "When Israel was a young man, I loved him like a son, and I summoned my son out of Egypt," Matthew claims that Jesus replicated this in a fuller way. Likewise, when God promised to David to claim David's successor as his own son to make a permanent dynasty, the author of Hebrews asserts that this is true of Jesus far beyond how it was true of Solomon.
3. God is the Father of those who trust in Jesus. One of the several ways the Bible describes the enormous salvation that God offers people in Jesus Christ is the staggering idea that people can actually become his children: "But to all who received [Jesus]—those who believe in his name—he has given the right to become God's children—children not born by human parents or by human desire or a husband's decision, but by God" (John 1:12-13). Notice that this privilege doesn't come from natural birth, which means that you're neither lucky enough nor too unlucky to receive that status based on who your parents happen to be. It also means that it isn't a privilege received merely by being born human. The universal condition for becoming God's child in this sense is trust in Jesus Christ his Son as Savior. As I've talked about before, faith in Christ is what unites us with Christ so that what is true of him (in this case, Sonship) becomes true of us.
Paul likes to describe becoming a part of God's family in terms of adoption with an eye to the authority and property that accrues to us as a result. For example,
For all who are led by the Spirit of God are the sons of God. For you did not receive a spirit of slavery leading again to fear, but you received the Spirit of adoption, by whom we cry, "Abba [Aramaic, 'Daddy'], Father." The Spirit himself bears witness to our spirit that we are God's children. And if children, then heirs (namely heirs of God and fellow-heirs with Christ)—if indeed we suffer with him so we may also be glorified with him [Rom. 8:14-17].John on the other hand talks about it as being "born from above" or "born again" (depending on how the Greek is translated)—a spiritual birth, begotten by God and receiving his "genes," in contrast with one's physical birth, begotten by one's earthly father and getting his genes (see John 3:3-7; 1 John 5:1), so that we actually become transformed into his divine nature.
4. God is the Father of all humanity. There are a tiny, tiny number of references in the Bible to God being the Father of all people. The only one that is indisputable in my opinion is Acts 17:26-29, where Paul argues to the learned citizenry of Athens,
From one man [God] made every nation of the human race to inhabit the entire earth, determining their set times and the fixed limits of the places where they would live, so that they would search for God and perhaps grope around for him and find him, though he is not far from each one of us. For in him we live and move about and exist [Epimenides, Cretica], as even some of your own poets have said, "For we too are his offspring" [Aratus, Phaenomena 5]. So since we are God's offspring, we should not think the deity is like gold or silver or stone, an image made by human skill and imagination.Paul, a Jew, is trying to establish common ground with the Athenians by identifying a single God as the one who oversaw the destinies of both their nations and that both have access to and even by quoting Greek authors to buttress his case. He is arguing that since we agree that humans sprang from God's creativity in his image, it doesn't make sense to worship a god sprung from a human's creativity in his image.
God's creation of humanity in his image that Paul argues from in Athens is the source of the "we are all God's children" concept, and obviously that's entirely legitimate. I do believe that this is worth talking about in order to establish common ground with people of other religions or no religion, especially those who have some "Only God" or "Chief God" concept. It is also important as a means to detach religion from ethnicity, culture, and/or citizenship, which is a crucial distinguishing mark of Christianity from some other religions like traditional Judaism. If we can agree that people of all nations are God's children and can "find him," then I can engage constructively with a Jew even though I'm not Jewish and with a Hindu even though I'm not Indian.
However, in our culture this genuinely biblical idea has been blown entirely out of proportion to its attestation in the Bible. A bit over a century ago, "old liberal" Protestantism in the West identified the Universal Fatherhood of God, the Brotherhood of Man, and the Infinite Value of the Human Soul (to paraphrase Adolf Harnack) as the essence of Christianity. As so often happens in these situations, generations down the line this rarefied idea suffused the popular culture.
But there are serious problems with making God's universal Fatherhood of the human race the primary or even sole understanding of his Fatherhood. Paul used the idea of "God, the Father of all humanity" as a bridge to telling Gentiles about "God, the Father of God the Son," so that in turn he could proclaim "God, the Father of those who trust in Jesus," which is really where he wants to end up (later instructing new believers as to how this fulfills "God, the Father of Israel," which is basically what his letter to the Romans is about). In other words, he was willing to talk about the family of God in the most inclusive sense to win a hearing for more exclusive senses of God's Fatherhood. Because without knowing about God as the Father of those who believe in Jesus, Paul's hearers could never become God's children in that sense.
Unfortunately, the concept of God's universal Fatherhood has inoculated people to understand, care about, or even be open to the fullness of what it means to be a child of God. The idea that Jesus Christ is the Son of God isn't striking to people who believe that everyone is a son or daughter of God. The idea that we can be adopted into God's family isn't awe-inspiring to people who think they are already in it. The unfathomable wonder of what it means to have the all-powerful Creator as one's real Father is lost to those who assume that in their run-of-the-mill lives they are already experiencing it. And if all people—apparently good, apparently evil, and everything in between—are children of God, then the concept is empty of the ethical implications that would be obvious otherwise (for example, "Be perfect, as your heavenly Father is perfect," Matt. 5:48; see also 1 John 3:10). This is yet another example of how when a little bit of truth gets blown out of proportion it becomes its own kind of lie and in fact is usually far more powerful than outright falsehood.
So, are we all God's children? In one sense, yes, we are. But in a sense more important to our eternal destinies, some people have become God's children while others remain estranged. As repellent as this idea is in our faux-tolerant culture, holding fast to this distinction is a necessary part of inviting everyone to cross it.