While reading Knowing God by J. I. Packer, I was struck by one section in particular. I quote it here in its entirety: 

"The revelation to the believer that God is his Father is in a sense the climax of the Bible, just as it was a final step in the revelatory process which the Bible records.

In Old Testament times, as we have seen, God gave his people a covenant name by which to speak of him and call upon him: the name Yahweh (‘Jehovah’, ‘the LORD’). By this name, God announced himself as the ‘great I AM’—the One who is completely and consistently himself. He is: and it is because he is what he is that everything else is as it is.

He is the reality behind all reality, the underlying cause of all causes and all events. The name proclaimed him as self-existent, sovereign, and wholly free from constraint by, or dependence on, anything outside himself. Though Yahweh was his covenant name, it spoke to Israel of what their God was in himself rather than of what he would be in relation to them. It was the official name of Israel’s King, and there was something of regal reserve about it. It was an enigmatic name, a name calculated to awaken humility and awe before the mystery of the Divine being rather than anything else.

In full accord with this, the aspect of his character on which God laid most stress in the Old Testament was his holiness. The angels’ song which Isaiah heard in the temple, with its emphatic repetitions—Holy, holy, holy, is the LORD Almighty’ (Isaiah 6:3 NIV)—could be used as a motto-text to sum up the theme of the whole Old Testament. 

The basic idea which the word ‘holy’ expresses is that of separation, or separateness. When God is declared to be ‘holy’, the thought is of all that separates him and sets him apart and makes him different from his creatures: his greatness (‘the majesty in heaven’, Hebrews 1:3; 8:1), and his purity, (‘Your eyes are too pure to look on evil; you cannot tolerate wrong’, Habakkuk 1:13). 

The whole spirit of Old Testament religion was determined by the thought of God’s holiness. The constant emphasis was that human beings, because of their weakness as creatures and their defilement as sinful creatures, must learn to humble themselves and be reverent before God. Religion was ‘the fear of the Lord’—a matter of knowing your own littleness, of confessing your faults and abasing yourself in God’s presence, of sheltering thankfully under his promises of mercy, and of taking care above all things to avoid presumptuous sins. Again and again it was stressed that we must keep our place, and our distance, in the presence of a holy God. This emphasis overshadowed everything else.

 But in the New Testament we find that things have changed. God and religion are not less than they were; the 0ld Testament revelation of the holiness of God, and its demand for humility in man, is presupposed throughout. But something has been added. A new factor has come in. 

New Testament believers deal with God as their Father. ‘Father’ is the name by which they call him. ‘Father’ has now become his covenant name—for the covenant which binds him to his people stands revealed as a family covenant. Christians are his children, his own sons and daughters, his heirs. 

And the stress of the New Testament is not on the difficulty and danger of drawing near to the holy God, but on the boldness and confidence with which believers may approach him: a boldness that springs directly from faith in Christ, and from the knowledge of his saving work. ‘In him and through faith in him we may approach God with freedom and confidence’ (Ephesians 3:12). ‘Since we have confidence to enter the Most Holy Place by the blood of Jesus, by a new and living way opened for us….let us draw near to God with a sincere heart in full assurance of faith’ (Hebrews 10:19ff). 

To those who are Christ’s, the holy God is a loving Father; they belong to his family; they may approach him without fear, and always be sure of his fatherly concern and care. This is the heart of the New Testament message."