Many Christians I know want to debate with Atheists whether or not God exists (even I am guilty of this); although this is admirable from a Christian point of view, it is a fruitless debate. What we see as evidence in the Bible, skeptics can easily claim that our book is biased and contradicts other religious texts on who God is.
The main issue here is pinning physical evidence against metaphysical evidence. In short, we cannot mathematically prove God. We can use science to advance claims of God, but we cannot prove the absolute existence.
Don’t be discouraged, however, as there is a much better way to stake your claim. With a lack of evidence of support, there is also a lack of evidence against. You have to realize that most Atheists have been told their whole education about why God cannot exist, or why He doesn’t. Of course, no real evidence is used (because of a lacking of evidence), and it is easier to just say, “no evidence, no God.” So, from a scientific standpoint, because there is no absolute way to verify the claim, it must remain neutral, as it cannot be proven nor disproven. This means that there is common ground, and leads way to agnosticism. Christians (sadly) often frown upon agnotisticism because those people claim that they are open to “a god” but have a lack of evidence to pursue one religion or deity over another. At that point, your life as a testimony can be used. It’s not a matter of converting people – it’s about being an example to them. No person has any ability to change another persons beliefs – individuals think for themselves and want to seek out information. John 13:34-35 says that people will know we follow Jesus by our love for one another. I can’t count the number of times I’ve heard stories (or had some of my own) that people know someone is a Christian intutively without the Christian asserting (by any means) their faith.
Bottom line – debating God is not evangelism. It’s trollism, and that doesn’t help anyone. Live how Jesus taught you to, and that’s the best evangelism you can have!