Quite often you will encounter people who will say that they don't believe in God.
Here is a good way to respond:
"Think about it like this: If you look at a bus, you know there was a busmaker. Or if you look at a building, you know there was a builder.
So when you look at the universe and since you know it had a beginning, it therefore must have had a Creator. Even though you've never seen the Creator, you still know He exists. Does that make sense?"
Try it, it works.
We've found that almost always people will agree with you. Why? Not just because it's logical, but because the Bible says that everyone already knows that God exists since God has shown it to them "through the things that have been made" (Romans 1:20). So this simple little apologetical argument simply reminds people about the God they know exists.
Then it's good to say:
"Since God exists, it matters therefore what we do here on this earth in that one day God will judge each of us based on how we have lived. So do you think you're a good person?"
This leads on to the good person test. Not familiar with that? Take a listen to a sermon about it here: https://www.livingwaters.com/hells-best-kept-secret
Now if a person is stubborn and blatantly disagrees with your argument for God's existence, it probably shows that he is being unreasonable, and so instead of remaining for too much longer on the apologetical side of things, it's best to continue on in the message - talking about the judgement to come and then the gospel - otherwise you'll never get to talk about it. For the gospel is the power of God for salvation (Romans 1:16).
Related Questions:
How do I respond to "What if my good deeds outweigh my bad deeds"?