Some Bible passages indicate that a sinner becomes a child of God by regeneration and other passages teach that he becomes a son of God by adoption. This does not mean that there are two different ways of becoming a member of God’s family. Rather, regeneration gives a sinner the nature of a son of God whereas adoption gives him the status of a son.
It is worth realising the distinction between justification and adoption. Justification deals with our failure to serve God perfectly. When we are justified, it means we are restored to the relationship of servants of God which we had at the beginning. That of course is a great privilege. But it is not as great a privilege as adoption. We can imagine God justifying us, forgiving us our sins, but not bringing us into his family. It was an additional act of grace to do so. Adoption is a higher blessing because of ‘the richer relationship with God that it involves’ (J. I. Packer). Or as James Buchanan put it, ‘This [adoption] is a higher privilege than that of Justification, as being founded on a closer and more endearing relation.’
It is also important to remember that adoption is not the same idea as God being the father of his creatures. The universal fatherhood of God is a teaching that is common in liberal Christianity. The Bible does teach that God is father by creation in the sense that he is the originator of humans, as Paul declared to the philosophers in Athens (Acts 17:29). But he is not their Father in the sense that they belong to his family. To appreciate this, we need to remind ourselves of what occurred at the Fall of Adam. Adam was made both a servant of God and a son of God. When he sinned, he became a rebellious servant and a prodigal son. ‘A man does not cease to be a subject, when he becomes a rebel; and no more does he cease to be, in some respects, a son, when he becomes a prodigal’ (Buchanan).