Ask them questions about what they believe, not to trap them, but to understand them and break down any relational barriers that are keeping them from considering Jesus.
Admire everything you can about what they believe in order to help people open up. Don’t affirm their heresy but their honesty. Find areas of common ground you can compliment them on.
Admit that the reason you are a Christian is that you know you’ve fallen short and need someone to rescue you. That someone is Jesus.