You need only read the book of Leviticus and then turn to Acts to sense the seismic change. Whereas Old Testament worshipers purified themselves before entering the temple and presented their offerings to God through a priest, in Acts God’s followers (good Jews, most of them) were meeting in private homes and addressing God with the informal Abba. It was a familiar term of family affection like “Daddy,” and before Jesus no one would have thought of applying such a word to Yaweh, the Sovereign Lord of the Universe. After him it became the standard word used by the early Christians to address God in prayer.
Earlier, I drew a parallel of a visitor in the White House. No such visitor I said, could expect to barge into the Oval Office to see the President without an appointment. There are exceptions. During John F. Kennedy’s administration, photographers sometimes captured a winsome scene. Seated around the President’s desk in gray suits, cabinet members are debating matters of world consequence, such as the Cuban missile crisis. Meanwhile, a toddler, the two-year-old John-John, crawls atop the huge Presidential desk, oblivious to White House protocol and the weighty matters of state. John-John was simply visiting his daddy, and sometimes to his father’s delight he would wander into the Oval Office with nary a knock.
That is the kind of shocking accessibility conveyed in Jesus’ word Abba. God may be Sovereign Lord of the Universe, but through his Son, God has made himself as approachable as any doting human father. In Romans 8. Paul brings the image of intimacy even closer. God’s Spirit lives inside us, he says, and when we do not know what we ought to pray “the Spirit himself intercedes for us with groans that words cannot express.”
We need not approach God by a ladder of hierarchy, anxious about cleanliness issues. If God’s kingdom had a “no Oddballs Allowed,” sign posted, none of us could get in, Jesus came to demonstrate that a perfect and holy God welcomes pleas for help from a widow with two mites and from a Roman centurion and a miserable publican and a thief on a cross. We need only call out “Abba” or failing that simply groan. God has come that close.

Phillip Yancy, What is so Amazing about Grace? Zondervan 1997, page 152