A friend of mine, Barry Coward, sent me and other Christians of his acquaintance, an email this weekend. In it, he explained that a local 12 year old daughter of friends of his had been asked to write an essay on "Why do Christians Believe in God?". It's been ages since I actually thought of something so foundational, so I wrote the following in a stream of consciousness... I'd be interested to hear what others would have said in the same circumstances.
Dear young, Lincolnshire friend of Barry,
That's a tricky bit of homework you have there!
For a lot of Christians, I think that this homework title seems strange. God tends to be the 'given' - the 'obviously there' thing that we just know, experience and accept already. I believe in God and am a Christian, but it's not that I came to the conclusion that God exists following a lot of reasoning-out, which seems to be the way the mainstream media, the British Humanist Association and even the RE curriculum imply I should!
All I can do to help is share my own experience of being a Christian and what I know of God from that.
Christians believe that God created humans in the image of God. This doesn't mean that God looks like a human - it's more about the kind of spiritual beings we humans are. In all human diversity, whatever our gender, nationality, culture, sexuality or age, there is something of God in every person. This is one reason why Christian morality focusses on loving others. Jesus taught that the most important rules to live by are to love God and to love each other.
The Incarnation - what Christians call the historical event of Jesus being born - is about God physically being on earth as a human being. This wasn't done because God somehow had to prove he existed, but was part of this same story, about a relationship between God and those who have been created to be like him: we didn't create God in our own image, as some perfect, never-dying kind of super-human. It was the other way round. God created us to be individual reflections of God, and to reflect all that is good, wonderful and God-like in ourselves to each other. Once you start to see other people in this way, and look for the Divine even in those who hate you, the imperfect world of humans starts to look more hopeful.
For Christians, the question of God's existence is closely linked to Jesus. The message of Jesus isn't so much his teachings - indeed, other religions have made similar points about loving God and each other. Rather, the point is about who Jesus is: the Son of God who was with God at the very beginning of everything and then was born and lived just like you or I at a particular point in human history. So it is because Jesus is God in human form, he himself is the message, and the message is "God is with us". Jesus is a huge reason why Christians believe in God.
In the end, though, it isn't a question of proof of God's existence, but of faith. St. Paul wrote "faith is being sure of what we hope for and certain of what we do not see." (Hebrews chapter 11 verse 1).
There's a huge amount I haven't written, and some people spend their lives writing book after book on this, but I hope this personal take on your homework helps.