Try your best to be respectful and listen to understand them when they talk. Repeat back their own words to make sure you understand what they are saying and sometimes they’ll even revise their position after hearing what they think. Allow for long pauses when they say something profound, and give space to the conversation so that they can feel safe when speaking about a deeply held belief.
Understand that the frustrations they might have are because they genuinely feel the way they do. And that because of how they feel they might also be worried for you for where you fit in their framework of the way the world works.
As an atheist, try your best to focus on the methods that we can know things to be true, not the beliefs themselves. Ask how confident they are, you might be surprised to hear that they are less confident than you think. And if they are quite confident, be genuinely open to their ideas, after all, if they are right, then it would be a very big deal. However suggest that before you believe, because you’d like to believe only true things, you’d like to know that the methods they are using are reliable. Try to stop the discussion if you get to an “I don’t know” so that they can process their new questions, and hopefully you can place a pebble in their shoe for deep discussions later on.