Julian of Norwich, in her Shewings or Revelations of Divine Love, compares the human condition - represented by the sin of Adam - with the relationship between a master and a servant:

The lord looks upon his servant very lovingly and sweetly and mildly. He sends him to a certain place to do his will. Not only does the servant go, but he dashes off and runs at great speed, loving to do his lord's will. And soon he falls into a dell and is greatly injured: and then he groans and moans and tosses about and writhes, but he cannot rise or help himself in any way. And of all this, the greatest hurt which I saw him in was lack of consolation, for he could not turn his face to look on his loving lord, who was very close to him, in whom is all consolation; but, like a man who was for the time extremely feeble and foolish, he paid heed to his feelings and his continuing distress.

The servant, desperate to do his master's will, runs off so excitedly that he slips and falls, and - so desolate is he at his failure to do what was asked - groans and writhes in his fallen condition, but is unable to pick himself up. But his greatest lack is his inability, in all his self-loathing, to turn to his master, for were he to do so, he would see his Lord next to him, stooping to help him up.

The story of the master and the servant speaks to us of the common experience of trying and failing, of wanting to do well, but being unable to do so. This is a helpful illustration of the Christian life. Julian spells out the essence of grace - that God gives himself freely to bring created humanity into closer union with the divine. And she does so by il- lustrating the practicality of the Christian life.

Grace is more than a word we use to fill in a gap between human failure and divine love. Christian theology maintains that that gap is indeed filled. But it is not filled by a concept, it is filled by an act - the loving act of God, which brings something from nothing.

Just as God's action in creation brings about those things we call real, the consequences of grace are real. In the most celebrated of St Thomas Aquinas's many writings, the Summa Theologica, the discussion of grace immediately precedes a discussion of the theological virtues, faith, hope and charity. The ordering is deliberate. Grace works itself out in human lives, by effecting real changes in real people.

The consequences of these changes are the dispositions to act in accordance with the will of God, which we call virtues. Faith, the first of these, is an act that is mine, but also God's, in that my ability to attain the intellectual knowledge of that which is infinitely beyond my capacities is possible only by grace.

This "knowledge" is not, however, the kind of knowing that applies to most of our experiences - I might "know" that 7 × 5 = 35, through some fairly normal means of human learning. Faith, according to Aquinas and others, consists in my being united with the object of my faith.

The grace that enables me to participate in the life of God becomes the vehicle for my "knowing" not a fact, but the source of all truth. Knowing God then enables me to "know" everything else in the light of this fundamental illumination.

The Revd Dr Peter Groves is Vicar of St Mary Magdalen's, Oxford.

This is the third of four edited extracts from  Grace: The cruciform love of God (Canterbury Press, £12.99 (CT Bookshop £11.70 - Use code CT277 ); 978-1-84825-054-3.