It is simply not the case that God has left us a lack of evidence. We simply find God's words distastful. Polly Toynbee and Richard Dawkins have both spoken out on this in the media in recent months. Notable atheists by confession. They have grasped the basic content of the Christian gospel. They see that the crucifixion of Jesus is central. However they look at it and call it "repugnant" and "barking mad". They are offended by such a message from God. As are countless others throughout time. Do not be fooled into thinking the gospel is unavailable or inaccessible, neither vague nor mysterious.I think however that faced with this question now instead of giving a quick answer I'd want to pose more questions:
The problem is not in God's revelation but rather in the human heart. A heart that looks at the grace of God and refuses it. It is human pride that blinds Toynbee to the truth. It is the sheer simplicity that so befuddles Dawkins. The Christian gospel is good news and bad news for us. It is our only hope of rescue, and simultaneously the defeat of our self-assertion and proudness of heart. Everything necessary has been done.
Shelley was evidently incensed by the gospel. Venomous and determined to refute it. He was not unaware of it. Like so many others he was offended. The Cross is not a message we wanted to hear. It did not fit the mould of a god he wanted to believe. Perhaps he never really saw the true gospel. Perhaps he never saw it truly lived out in the Christian community. His questions were good and honest. My answers are too late for him, hopefully not for others.
1. If God isn't obvious, what is obvious when you look at the world?
I guess this might lead to some comment about the apparent futility of the world - in suffering etc in which we'd find CS Lewis a good friend to introduce into the conversation.... A genuine sense of the wrongness of wrong is already a substantial step towards theology.
2. Why do you think Christians think it so necessary to know God?
What is it that Christians you've met seem to think is so good and beneficial and worth having about Jesus? This might lead us to talk about how he simply doesn't seem worth getting to know, or that he's just a ticket out of hell... but it might also allow us to talk about some of the differences in the lives of Christians, the importance they place on Christ, the way they treasure him not because he can get them out of hell but as an end in himself.