Reason #1: Looking at the world and being disappointed.
Reason #2: Creation.
Reason #3: The cross.
Reason #4: The transforming love of God.
I listened to a sermon at my church a few weeks ago and was struck by one sentence which I jotted down immediately.
The Christian has experienced God's love and has been transformed by it.
This feels like an appropriate reason to follow Reason #3, the cross of Christ, an event and a symbol of God's great, self-sacrificial love for us. As Romans 5:7-8 says, "Very rarely will anyone die for a righteous man, though for a good man someone might possibly dare to die. But God demonstrates his own love for us in this: While we were still sinners, Christ died for us."
But it doesn't stop with the cross as an event. The power of the cross is the power of a love so great that it is tangible to me, a 23-year-old in 2010, and it has been tangible to me throughout my life. The love of a God I can turn to again and again, a God who forgives me again and again, who continues to bless me despite all my failings; a God who saw me running away, fought for me, and ran to greet me with open arms as I stumbled back towards him; a God whose love has been so immediately present, so real, so comforting, in all the most horrible moments of my life.
It's not something I can quantify because it is so all-encompassing and it's so personal. I cannot prove to you the depth of God's love. A frail example would be like trying to prove that my mother loved me - unmeasurable by the fact that she fed me, clothed me, looked after me when I was sick, although that is all part of it.
However, it is something that I have seen in other people's lives, and the proof is in the transformation. I've seen proud, strong, angry people brought to their knees by God's love; men who used to drink and fight and hit their wives, lives transformed, stand before a congregation every week and declare "God is so good"; people whose actions led them to be rejected by society come in humility and shame before a God who forgave them. I've seen this in people I know, and I've seen it in me, and there is no explanation for it that does not take into account a force outside of these people, a God who loves them.