As stated before, Atheists are not people that believe in gods. They lack any kind of god whatsoever. This doesn't mean they reject the idea of gods, they just don't believe in any particular god. Why? The most used reason is that there is no evidence for the existence of any known god. This reason usually is not enough for Theistic believers, as it usually prompts a very common question.
"What do Atheists define as evidence?"
You may be surprised at how often this question comes up. Most Atheists are actually surprised that this question is even asked. As far as they knew everyone used the same definition of evidence. However, there are many people that do not look up definitions in dictionaries until pressured to. As such, there are many people using words in the wrong way. To keep things simple, let's look at the dictionary definition of Evidence.
That which tends to prove or disprove something.
That's a pretty simple definition. Evidence is something that proves a claim or disproves a claim. With this definition in mind, most Atheists agree that we currently have no evidence to prove or disprove the claim of gods. This is why there are so many Atheists in the first place.
Theists continue to claim there is evidence for their god(s) and that it is sufficient to prove the validity of their position. However, this simply is not the case. The most evidence that any religion currently has is their holy scriptures, which themselves do not have much credibility or evidence supporting them. Usually this is countered by claiming that their scriptures were divinely inspired, which is not evidence of anything or evidenced by anything.
On the opposite side, it is impossible to provide evidence for a negative claim, only negative evidence to a positive claim. As such it is impossible to prove that no god exists. This is why most Atheists hold the position of lacking evidence for the positive claim that any particular god exists.
So, the best way to convert an Atheist is to provide evidence for your position. And Atheists need to research their own position, to make sure that provided evidence, on both sides, is accurate.