I think a lot depends on the person. Are they a thinker? or a feeler?
With the thinker you may need to start with a discussion about what is “truth.” Can it be relative and still be truth? Abdu Murray’s book called “Saving Truth” is a good resource.
If they are a feeler, then what do they feel? Do they have a sense of something larger than themselves? If so, what might that be? How do they feel personally…inside? Do they ever feel like if people really knew them then no one would ever love them? That can be a great opening to the love of Christ.
If they believe in a higher power or intelligent source then what might be implied with that belief? If there’s a creator, then is it reasonable to expect that the creator might have expectations of them? Are they qualified to judge their creator? Is anything due to the creator? What if they created something, would they want to just sit it on a shelf and then ignore it…or would they want to interact with it? How? What would they want?
There’s many ways to take a discussion. Sometimes we need to discover where our ideas and feelings come from and how they might not really be rational when you shine a light on them.
When a person is negative about who God is, then there’s likely a source for that negativity. Have they really ever looked into what a Christian believes about the character of God?
Are they willing to understand that a perspective from within may be entirely different from an external perspective? C.S. Lewis contrasts viewing a beam of light from outside it and viewing the source of the light from within it–the viewers are seeing entirely different things. This kind of thing can be common ground where a discussion can bear fruit.
Be relational. Value them and their opinions and build a friendship. Be consistent and let them see you reflecting a set of values that is consistent with your beliefs. Give it time. Pray. Have fun with the person. Delight in them. Let them see the love of Christ through you.
Sometimes it takes a lot of work to go from an ambivalence to the truth to caring about the truth and then to seeking the truth and hopefully, ultimately, knowing the truth.