J. I. Packer:

You sum up the whole of New Testament teaching in a single phrase, if you speak of it as a revelation of the Fatherhood of the holy Creator.

In the same way, you sum up the whole of New Testament religion if you describe it as the knowledge of God as one’s holy Father.

If you want to judge how well a person understands Christianity, find out how much he makes of the thought of being God’s child, and having God as his Father.

If this is not the thought that prompts and controls his worship and prayers and his whole outlook on life, it means that he does not understand Christianity very well at all.

For everything that Christ taught, everything that makes the New Testament new, and better than the Old, everything that is distinctively Christian as opposed to merely Jewish, is summed up I the knowledge of the Fatherhood of God.

Evangelical Magazine 7, pp. 19-20, cited in Knowing God, p. 201. Packer says on p. 202:

Our understanding of Christianity cannot be better than our grasp of adoption.