Sons Of God

What is a Christian? The question can be answered in many ways, but the richest answer I know is that a Christian is one who has God for his Father.
But cannot this be said of every man, Christian or not? Emphatically no! The idea that all men are children of God is not found in the Bible anywhere. The Old Testament shows God as the Father, not of all men, but of His own people, the seed of Abraham, 'Israel is my son, even my firstborn: and i say into thee, Let my son go...' (Exodus 4:22).
The New Testament has a world vision, but it too shows God as the Father, not of all men, but of those who, knowing themselves to be sinners, put their trust in the Lord Jesus Christ as their divine sin-bearer and master, and so become Abraham's spiritual seed. 'Ye are all sons of God, through faith, in Christ Jesus... ye all are one man in Christ Jesus. And if yes are Christ's then are ye Abraham's seed' (Galatians 3:26).
Sonship to God is not, therefore a universal status upon which everyone enters by natural birth, but a supernatural gift which one receives through receiving Jesus. 'No man cometh unto the Father' - in other words, is acknowledged by God as son - 'but by me' (John 14:6).
The gift of sonship to God becomes ours, not through being born, but thorough being born again. 'As many as received Him, to them gave He power to become the sons of God, even to them that believe in His name: which were born, not of blood, not of the will of flesh, nor of the will of man, but of God' (John 1:12)

J. I. Packer, Knowing God