When we use the word "God" we are engaging in theology, and it is sobering to remind ourselves that theologians are, by definition, those who do not know what they are talking about.

The reason for this is simple. God, according to Christian teaching, is the creator. Everything that exists owes that existence wholly to God. When we talk of God, we are creatures using created things - words - to point towards that which is infinite and ineffable. If I attempt to define God, I necessarily limit him. (Even using the pronoun "him" is a limitation, as if God could be defined by gender.)

To limit the creator to that which we know from creation really makes no sense. We understand things in the world by reason and by experience. We observe patterns of behaviour; we conclude what is true about the nature of the world through science, we learn about ourselves and those with whom we interact through our thoughts and actions. But what direct "experience" do we have of God? The ways in which we speak of God are indirect.

Let's take the example of emotion. Human beings are often heavily influenced by their emotions, and those emotions provide us with some powerful descriptions - anger, rage, grief, desolation, for example. But human beings are limited, physical agents.

We can say, and scripture does say, some remarkable things about the love of God, using the language of emotion. But this language will always be partial. It is the physical nature of humanity that gives it emotion. So it would be dangerous to assume that the same sort of language with which we express human emotions can define the God who created those human beings in all their physical limitation.

The question "How can I know God?" is basic to humanity, we could argue, and it is a question that is answered clearly in Christian teaching with a name: Jesus of Nazareth. The extraordinary claim of Christianity is that the infinite and unknowable creator comes among us as a person and transforms humanity by drawing it up into a relationship with himself. "No one has ever seen God. It is God the only son, who is close to the Father's heart, who has made him known" (John 1.18).

This central notion - that God is known to us only because, in the free gift of his grace, he chooses to make himself known - is a model for Christian teaching on the subject of grace. Grace is what God is like - the infinite generosity that is God's self-giving love - and grace is God's gift to enable us to be like him, to unite our lives with his, and so it describes the particular characteristics of Christian life which make possible the imitation of the divine to which every Christian is called.

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

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