One thing to consider, or to share with an atheist/agnostic friend/foe, regarding the existence of God is something I like to call "the orphan thought experiment".
The thought experiment goes something like this. Imagine that you are an orphan. You've never met your parents and you know nothing about them. You live in an orphanage. Now, the fact that you've never met your parents and that you know nothing about them wouldn't lead you to believe that you don't have parents at all, would it?
In a related way, given what we are as a species, it makes little logical sense to imagine/believe that we just suddenly appeared out of nowhere; even if the "appearing" takes place in the form of what people call evolution. God may well have created man through some process which we don't well understand and might think of as "evolution" or which looks like evolution to us. Regardless of how He created us, to imagine that it all happened without God as Father (eternal parent) would make as much sense as an orphan imagining that he/she just suddenly appeared out of nowhere.