To start with, it’s always easier to work out what to say when there’s no-one actually responding to you (as with me now). I, as a participant in the conversation, cannot dictate how the conversation will proceed. When I make a statement, everything depends on the answer I’m given. It’s easy to start heading for one conversation while ending up with a totally different one. That said, I think I would try something like this:
When I was a child, we all agreed together about what was right and what was wrong. We didn’t always do what was right, but we agreed together that when we behaved like this, we had done something wrong. Stealing was wrong, as was lying. Being kind and helpful was good. (Although we didn’t know it, this had the idea that God has said what is right and what isn’t behind it.)
However, now our society has drifted to the position where everyone is right. We think that there are no absolutes. I think it’s hard to argue for absolutes without God. But if God is there, then that makes all the difference. We should be asking the question “How can I get right with that God?”
Another answer could go like this:
Have you ever thought about why people seem to be basically selfish? No-one but a selfish person would write graffiti. That person doesn’t care about our public property; he or she only thinks in terms of my private property. I wonder why we are like this. People seem to be selfish from birth. No-one needs to teach us. Something is radically wrong. The Bible says it’s because we’ve all turned away from God and gone our own way. We need that trend to be reversed. Only God can do that.
Having read what I have written, I realize that they are all long answers. In conversation, it isn’t like this; I say something and they say something, and what they say will govern what I can say next (unless I ignore them altogether, which won’t get me very far). My answers show where I am trying to get the conversation to go because I can get to the gospel easily then.
Let me try and envisage how a conversation might go:
Other person: Why is there so much graffiti in this bus shelter? Why don’t people respect public property any more?
JCC: When I was a boy, we all agreed that spraying graffiti is wrong, but not any longer.
OP: You mean some people think it’s okay?
JCC: I guess so. I think if a person doesn’t believe in God, it’s hard to always know what is right and wrong.
OP: I don’t believe in God and I know that this behaviour is wrong.
JCC: Really? What makes it wrong? Who should say what is right and wrong? And why …?
My aim is to ultimately show that we can’t live in society without absolutes. From there, I would argue for the existence of God who is the author of absolutes, and from there, to how I can get right with him.
In any given conversation, I would pray all the time, asking God to help me to say what is helpful—what will lead towards the gospel and not away from it.